The AirGun Guild

General Category => Break Barrel, Pumper and Springer Airguns => Topic started by: rsterne on December 25, 2019, 03:25:15 PM

Title: My Benjamin 392
Post by: rsterne on December 25, 2019, 03:25:15 PM
I have never had a Benjamin 392 pumper before, and Eric at Scopes and Ammo was clearing them out last fall, so I had my wife pick one up for me for Christmas, and I opened it today.... She banished me to the shop to stay out of her way for a couple of hours this afternoon, so I did some baseline work to see what I am starting with.... As you can imagine I don't plan to leave it stock....  ::) …. Here is the velocity at each number of pumps, using the new 15.9 gr. JSB Hades pellets, which I thought might be a good weight for it....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Benjamin%20392%20Velocity%20Stock_zpsokfazt91.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Benjamin%20392%20Velocity%20Stock_zpsokfazt91.jpg.html)

What really surprised me is that it retained a tiny puff of air at 7 pumps, and enough at 8 pumps to deliver a shot of over 200 fps.... By the time I had pumped it 10 times, the second shot was over 450 fps....  :o …. This means that at 8 pumps and over, some extra power is lurking inside.... I know the valve spring is pretty stiff, it may only take replacing that with a weaker one to find that lurking FPE....

Next I used 8 pumps and tried a wide variety of pellet weights, from the 11.9 gr. RWS Hobby all the way up to the 34 gr. JSB Beast.... I shot 2 shots and recorded the average, and between every shot I cocked the gun and fired it to dump all the air.... The "puff" of air of the 2nd shot was minimal with the lightest pellets, but increased with pellet weight, as you might expect.... The fact it is retaining air indicates insufficient dwell to dump it all before the pellet reaches the muzzle, and heavier pellets, going slower, haven't moved as far from the breech, so the valve should have released less air (and therefore retained more)…. Here is a plot of velocity and energy vs. the weight of the pellets....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Benjamin%20392%20Stock%20Weights_zpshlhmmguk.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Benjamin%20392%20Stock%20Weights_zpshlhmmguk.jpg.html)

The behavior was much as expected, with the heavier pellets having more FPE.... What was interesting was that the rate of increase of FPE changed drastically at a pellet weight of about 19-20 gr.... Below that, you gain 2 FPE (17%) by increasing the pellet weight from 11.9 gr. to 18.1.... but above that the gain is less than that (1.8 FPE = 13%) at 34 gr.... You are still gaining energy with heavier pellets, but considering that the velocity is dropping below 600 fps, IMO there are two reasons not to use pellets heavier than about 20 grains.... BTW, I didn't have a pellet between 25.4 and 34 gr., so I used a 29.6 gr. BBT slug, as cast, and it fit right into the data.... This shows that a slug can deliver velocity and energy exactly the same as what you would expect from a pellet of the same weight....  8)

Anyways, this is a good baseline to have so that I will know what is happening when I change things.... I think the first change will be an O-ring spacer inside the valve to reduce the headspace, and likely the 2-spring mod. to reduce the pumping effort, or simply a lighter valve spring....

Bob
Title: Re: My Benjamin 392
Post by: steveoh on December 25, 2019, 05:42:22 PM
Good stuff!
Title: Re: My Benjamin 392
Post by: Alan on December 26, 2019, 05:32:37 AM
I have a 397, which according to Benjamin, can develop 10 FPE. No matter what I did, I couldn't get the velocity/weight combination to get over 8.5 FPE. However, a simple mod I found on the net (modifying the valve slightly), pushed it over 11 FPE. So there is hope for you Bob!
Title: Re: My Benjamin 392
Post by: rsterne on January 02, 2020, 01:15:50 PM
Somebody asked what my 392 looked like.... Here it is the way it came out of the box on Christmas morning....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Benjamin%20392%20Small_zps9nwt1eak.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Benjamin%20392%20Small_zps9nwt1eak.jpg.html)

I have a spare valve, so I pulled it apart and installed the 2-spring mod.... I also unscrewed the valve a bit, leaving a gap between the two halves which will decrease the headspace between the rubber cup on the piston and the tapered end on the valve.... Here are the photos....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/New%20O-ring_zps1fabxvno.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/New%20O-ring_zps1fabxvno.jpg.html)

Above shows the gap in the valve, which I initially set at 1/16".... In the lower photo you can see the stock valve spring, which pushes against the end of the check valve.... There is a shoulder down inside the valve front, just behind the back of the check valve, and a #8 flat washer will drop in and stop against that shoulder.... I used a much lighter spring in front, and turned the end of the check valve down to create a spring seat.... I had to do some work on the spring with a pair of needle-nosed pliers to create a taper in it to prevent it working its way through the hole in the washer....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/New%20Springs_zpsshdtxuuo.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/New%20Springs_zpsshdtxuuo.jpg.html)

The new valve spring is a LOT lighter, only 0.035" wire and 1.75" long instead of the stock spring, which is 0.049" wire and 2.13" long.... Stand by for a report on the results (not what I expected)….

Bob
Title: Re: My Benjamin 392
Post by: rsterne on January 02, 2020, 02:24:16 PM
So, to continue.... yesterday I disassembled the gun for the first time, and found to my surprise that it had the older brass valve.... I guess that dates it a bit, but dimensionally the valves seemed identical.... I was really not happy with how hard it was to drive out the roll-pins on the front plug, and it's even harder to get them back in straight, because they are drilled offcenter on the tube (the front ones high and the back ones low)…. I will be making steel shear pins to replace them, because I will be taking this gun apart so many times the brass tube won't survive the continual driving of the pins back and forth....

Incidently, I did a safety study of the parts at 1500 psi, and it looks like the limiting factor for pressure loading is the single #12-36 valve mounting screw.... Even though the 0.880" OD x 0.053" tube is only brass, the hoop strength is decent, and even the roll-pins are comparable (because there are 4 shear planes)…. compared to the loading on the edge of the tube at the valve screw....  ::) …. I don't know what the pressure is at 8 or 10 pumps (I may install a gauge to measure it)…. but past about 1200 psi the brass tube at the back end of the single hole where the valve is attached (the front trigger mounting) may start seeing distortion.... This isn't a "sudden" failure mode.... but should according to calculations be an indicator that you are pushing things too hard.... It makes me curious what a "Steroided 392" at 14-16 pumps may be seeing, and if any distorting is showing up at that valve screw hole....  :o

Anyways, on reassembly with the aluminum valve with the 2-spring mod. I found out that the hammer spring was holding the valve open, so I couldn't pump it.... I think this was causing the hammer to jam on firing (the hammer moving too far forward and catching on the forward bump on the sear)…. so I could not proceed further.... I pulled the gun apart again (cursing those roll-pins again, I really have to change those out)…. and disassembled the valve and found another valve spring, in between stock and the one I tried.... Here are the 3 springs....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Valve%20Springs%20Small_zpsixrn5ifz.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Valve%20Springs%20Small_zpsixrn5ifz.jpg.html)

The bottom one (on the poppet) which I am going to try next, is the valve spring from an MRod (I think)…. It is 1.43" long and made from 0.044" wire.... The crude testing I did indicates that it should just hold the valve closed against the stock hammer spring.... Here is the comparison of the current 2-spring setup and the stock 392 valve spring....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/2%20Piece%20Spring%20Small_zpsgiptwe0b.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/2%20Piece%20Spring%20Small_zpsgiptwe0b.jpg.html)

Oh, I almost forgot, with the 1/16" gap in the valve there was quite a bit of pressure on the pump arm just before it closed.... The rubber cup was hitting the (now further forward) end of the valve when the handle was nearly 3" from closed.... I will screw the valve a bit further together, leaving about a 0.045" gap next time.... Now to make some new pins for the front block, and reassemble the gun.... Hopefully this time it will pump and fire....  ::)

Bob
Title: Re: My Benjamin 392
Post by: Alan on January 03, 2020, 05:19:21 AM
I'm not an expert on the 39X valve variations (aluminum vs. brass), but Crosman told me several years ago, that the aluminum valve was only used for a couple of years. I bought my 397 almost 5 years ago, and it has a brass valve.

The valve "mod" I bought off eBay, uses a different spring (I didn't measure the differences between it and stock). And, the plastic (Nylon?) valve is shorter overall (≈.125"). At the same time, I installed a SuperSear from AoA, which turns out to be the best thing I did! Makes the gun much easier to fire. The stock trigger setup was horrible to say the least.

Before the mods, the best I could do, shooting the RedFires (7.8 gr.) was ≈675 FPS at 7 or 8 pumps. After the mod, the velocity was ≈780 at 6 or 7 pumps, and a bit less with 8 pumps. It shoots best (accuracy wise) with 5 pumps, at ≈750 FPS.
Title: Re: My Benjamin 392
Post by: rsterne on January 03, 2020, 02:14:01 PM
I made a pair of shear pins for the front of the pump tube today.... They are short pieces of 3/16" drill rod, threaded both ends with 10-32 thread.... On one end a nut is glued and pinned in place.... No more wearing out or damaging the brass tube wailing on those roll-pins....  8)

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/392%20Valve%20Tool%20and%20%20Pins%20Small_zps6xkg4z5z.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/392%20Valve%20Tool%20and%20%20Pins%20Small_zps6xkg4z5z.jpg.html)

The other item is a tool to rotate the valve when installing it so that the screw hole (and therefore the transfer port) line up properly.... In case you didn't know, that is why the 39X valve has a small hole in the back on the right side of the valve stem.... The tool is simply a piece of 3/4" OD 6061-T6 aluminum bar stock, with a 1/2" deep 3/16" hole in the center to clear the valve stem, and a 3/8" deep, 1/16" hole that is 0.21" off center with a piece of 1/16" piano wire glued in.... The screw is simply a handle on the bottom to allow you to turn the valve so that the threaded hole in the valve for the trigger/valve screw lines up with the hole in the tube....

Bob
Title: Re: My Benjamin 392
Post by: Alan on January 03, 2020, 03:12:23 PM
You sort of struck a chord with me (Robert), so I got out the 397 and the Chrono. Well.... I can still get almost 800 FPS with the RedFires, but it takes one more pump. Perhaps a seal as I haven't fired the gun for about 10 months.

Opinion: I think too many pass up pump guns because they think they're inferior to PCP guns. That fact I won't challenge. However, I have an after-market adapter which allows me to attach a suppressor to the 397. With it, the 397 is VERY (!) quiet, to the point, my next door neighbors don't know I have fired it, even if they're in their back yard, less than 10 feet away! I just love the stealthiness it offers when an errant squirrel just happens to come around.

Each and every airgun has a nitch. I've found my 397's nitch, and I suspect you will find one for your 392.
Title: Re: My Benjamin 392
Post by: rsterne on January 03, 2020, 03:57:57 PM
Now that I have the new pins and the valve tool, I can reassemble my 392 without fear of damaging it.... It looks a bit clunky with the nuts on the front pins.... but after the gun is finished (are they ever REALLY finished? ) I could make something a bit prettier.... Anyways, I reassembled the gun with the 2-spring mod installed, and the two halves of the valve with a 0.045" gap.... The pump arm now closes to within 1.5" of the tube after a shot (pump tube empty), so the rubber cup is being compressed against the end of the valve just a bit.... It turns out that the new valve spring is not QUITE strong enough to allow me to pump the gun without cocking it.... It is really close, so close it might work when I install the MRod poppet, which is 1/16" thicker, so will compress the valve spring more.... It's not an issue to have to cock the gun before pumping (unless you forget)…. all that happens if you don't, is you hear the air farting out the muzzle....

I checked the velocity with the 15.9 gr. pellets at every pump, and continued until the valve retained a "puff" of air.... Instead of that happening at 7 pumps, it now happens at 12 pumps, which shows what a difference the valve spring can make.... The gun is pretty hard to pump at 12 pumps, I don't think I will be doing that on a regular basis.... but at least I now know that it is not retaining air at 10 pumps, which it certainly did before.... Here is the previous chart, with the new data on it for the velocity and energy, shown as dotted lines....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Benjamin%20392%20Velocity%202spring_zpslg6mbekh.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Benjamin%20392%20Velocity%202spring_zpslg6mbekh.jpg.html)

From 7 pumps and up, the velocity and energy have increased, now that the gun is not retaining air.... I tried a few pellet weights, again at 8 pumps, to see what the gains were in velocity and energy, and the gun gained more with heavier pellets.... However, the inflection point in the curve remains at about 19 gr.... above that weight the FPE increases slower as the weight is increased.... That may change with larger ports.... and it may also change with increased valve volume....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Benjamin%20392%20Weights%202spring_zpsupdzvqmb.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Benjamin%20392%20Weights%202spring_zpsupdzvqmb.jpg.html)

I'm pleased with these modest gains from such simple mods that anyone can do.... I guess the next step is to increase the flow, by fitting an MRod poppet, and drilling out and smoothing the throat and ports.... Stay tuned....  ;)

Bob
Title: Re: My Benjamin 392
Post by: rsterne on January 04, 2020, 06:40:00 PM
Work progresses slowly on my 392.... I wanted to fit an MRod poppet, which has a smaller stem, 1/8" instead of 5/32".... At the same time, I decided to enlarge the valve throat from 7/32" (0.219") to 15/64" (0.234")…. This will increase the throat area from the equivalent of a 0.155" hole to that of a 0.198" hole.... which is a 63% increase in area.... I also wanted to enlarge and angle the exhaust port to ease the flow from the throat into the exhaust port.... Part of the plan to install the MRod poppet was to bush the hole in the back of the valve with K&S brass hobby tubing, but Charles (Psipumper) suggested that I drill the back of the valve straight through at 0.234" and then press in an insert which was drilled 1/8" for the stem of the poppet.... By grinding an angle on the front of the insert, it would fill some of the vacant space from the original vertical exhaust port, and the volume behind that in the throat....

Charles suggested that I drill the hole for the stem first, to insure it is straight and square to the valve seat.... but since I have a lathe, and I have made lots of my own valves, I figured I could drill the hole afterwards from the back.... First, however, I decided to drill and tap holes in both sides for 8-32 low profile SHCSs to better locate the valve in the tube.... I was concerned about the single valve screw, not so much for the strength of the screw itself, but the load on the thin brass tube.... I calculated the safety margin on the support given by the tube wall to that screw, which was the only thing keeping the valve from sliding back.... To my surprise, I found that at 1200 psi it was only 1.02:1....  :o

That means that if the pressure exceeds 1200 psi, you can expect the screw to start deforming the brass tube on the back of the hole.... IF that happened, the valve would move back, and the exhaust port would no longer line up with the port in the receiver.... Since it is always nice to have a 3:1 safety margin, I decided to add a screw to each side of the valve, set down against a flat spot milled in the valve body.... Once I drill holes in the sides of the brass tube, I will triple the ability of the brass tube to prevent the valve moving back.... The holes are in the same plane as the port and existing valve screw, and the head is slightly smaller than the countersink for the valve screw.... so they won't interfere with the installation of another O-ring ahead of the screws, in case I want to skeletonize the valve....

I set the valve up in a square indexing block (against an end stop) with a 5C collet, using a #20 drill to line up the exhaust port with the headstock.... Then by simply turning the block I could drill the side screw holes in the same plane and at 90 deg. to it and the existing valve screw hole.... Once those were drilled and tapped, and the flats for the heads milled, I turned the block so that the exhaust port was lined up again with the headstock.... I then rotated the milling attachment 20 deg. and plunged a 5/32" mill through at the location of the existing exhaust port.... I examined the inside to confirm that there was lots of room, changed the angle to 25 deg. and milled the front of the angled port.... I then raised and lowered the valve 0.012" to widen the exhaust port and make the hole where it meets the receiver round.... It ended up at 3/16"....

I removed the valve from the collet block, and chucked it in the lathe, with the exhaust port lined up with the chuck key hole.... That means I can put it back into the 3-jaw chuck in the same orientation each time.... I used a 3/16" stub drill and drilled along the hole for the stem, from the back of the valve.... I then went to 7/32", which was the size of the existing throat (it lined up perfectly), then a #1 drill (0.228") and finally the 15/64" drill to finish it to the size of the new throat.... I then made an insert out of 6061-T6 aluminum, about 0.002" over, ground an angle on the front (to match the exhaust port) and heated up the valve body to press in the insert.... Long story short, it didn't work, and stuck about halfway in.... I had to cut it off, machine it off flat with the back of the valve, and then drill it out again.... There was no damage to the valve, so I made a new insert, the same size as the hole in the back of the valve.... polished it a thou smaller and then pressed it in with Loctite 638.... It ain't going anywhere....  ;)

I then used a small spherical burr in my Dremel, and working through the exhaust port, smoothed out the front of the insert and faired the exhaust port into the throat.... Now, when you look into the exhaust, you can see part way through the throat of the valve, and the passages are nice and smooth.... An 11/64" (0.172" drill) wobbles around loose in the exhaust port, so it is larger than that.... The last step was to drill the 1/8" hole for the stem of the MRod poppet.... Here is the finished back half of the valve, along with the MRod poppet and spring....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Porting%20Screws%20and%20MRod%20Poppet%20%20Small_zpskxexbdt2.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Porting%20Screws%20and%20MRod%20Poppet%20%20Small_zpskxexbdt2.jpg.html)

The insert in the back of the valve covers the 2 side screw holes, and the original lower valve screw hole, sealing them off.... They are only under pressure during a shot, so no danger of the valve leaking down anyways.... One added bonus of the insert is that it supports a greater length of the valve stem, so it doesn't wobble around like the stock one did.... I haven't drilled the holes in the tube for the side screws yet, nor machined the extra O-ring groove.... but I think I will test the gun at this stage.... I have not yet drilled out the receiver/barrel port, nor turned down the bolt probe, so I will be able to see what difference that makes, if any, after testing the new valve port and poppet.... Incidently, I think there is room to angle the exhaust port 30 deg. instead of 25.... but it might get pretty close to the valve seat.... BTW, the MRod poppet seems to seal up perfectly, just blowing in the front of the valve it has no leak detectable....

Bob
Title: Re: My Benjamin 392
Post by: rsterne on January 04, 2020, 09:32:25 PM
Here is the testing with the valve mods so far.... larger port, throat, and MRod poppet.... I am only checking the velocity every 2 pumps.... The MRod poppet and valve spring, sitting on a #8 washer on the shoulder in the valve front, is just about perfect for this gun, IMO.... The poppet is smaller in OD, it has a smaller stem, the gun is easier to cock, and you can still pump the gun without having to cock it....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Benjamin%20392%20Velocity%20Valve%20Mods_zpsoswpijcc.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Benjamin%20392%20Velocity%20Valve%20Mods_zpsoswpijcc.jpg.html)

As you can see, the improvements continue.... In addition, I can now pump 14 times without retaining air.... I didn't try more than that....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Benjamin%20392%20Weights%20Valve%20Mods_zpsohwvmd4q.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Benjamin%20392%20Weights%20Valve%20Mods_zpsohwvmd4q.jpg.html)

I am no longer going to test anything lighter than 14.3 gr. pellets.... and I am thinking about pellet weight in the reverse, compared to before.... Instead of not looking heavier than 19 gr. because the FPE doesn't increase as fast, I think I should not be looking at pellets lighter than 15.9 gr. because the FPE deceases quicker.... Sometimes I get it backwards, and this is one of those times....  ::)

The gun is now producing 21 FPE at 12 pumps with 25.4 gr. pellets....  8) …. The next set of mods is to drill out the receiver and barrel port, which is now the smallest port in the gun.... While I'm at it, I will slim down the bolt probe as well.... I think that will complete the "breathing" mods, then I will take a look at the pump.... or, I may install a pressure gauge....

Bob
Title: Re: My Benjamin 392
Post by: Alan on January 05, 2020, 05:02:14 AM
Question remain... How many would go to this amount of upgrade work? And of course, still want to pump it after you're finished?

I like my 397, no more than I have changed it over stock. The main reason is, it is dead quiet. After you're finished, I'd like to know how much the report is??
Title: Re: My Benjamin 392
Post by: rsterne on January 05, 2020, 08:28:30 AM
It's getting louder, for sure.... but that is mainly a function of how much you pump it.... Remember, in Canada, quiet guns are not allowed....  ::)

Bob
Title: Re: My Benjamin 392
Post by: steveoh on January 05, 2020, 09:01:44 AM
Bob, you never cease to amaze me with your skills, creativity and enthusiasm. Then there’s your willingness to share your findings and knowledge with us. Thank you!

Title: Re: My Benjamin 392
Post by: rsterne on January 05, 2020, 12:53:01 PM
It would be hard to come up with something that hasn't been done before on a 392.... and the limitations of the plartform (eg. soldered barrel and receiver) make major (eg. caliber) changes a challenge, which is why I decided to stay with the basic .22 cal platform.... I doubt if my gun will be a record setter in any way.... but it's a fun project, and few have documented each step in their mods....

Bob
Title: Re: My Benjamin 392
Post by: rsterne on January 05, 2020, 03:55:35 PM
Here is the start of my preload adjuster, which may morph into an SSG.... I cut off the top of the trigger, which formed the safety interlock with the bolt.... This is designed to prevent firing the gun with the bolt open, but also meant that you could not uncock the gun, a feature I really didn't like.... I then took a 1.25" length of 3/4" OD 6061-T6 aluminum bar stock drilled a 21/64" hole through it so that it could be tapped to 3/8"-24NF for an adjusting screw, and then faced the ends off so that it was a drop-in fit into the recess in the top of the trigger housing.... It is a perfect fit, providing you get the length right.... Note that the potmetal casting has a slight taper to allow it to come out of the mould, so you have to measure the length at the bottom of the recess, not the parting face.... and on my trigger housing one side was about 0.005" shorter than the other....  ::)

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Adjuster%20Plug%20Small_zps4og5dlkb.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Adjuster%20Plug%20Small_zps4og5dlkb.jpg.html)

I will drill out the ends of the housing at some point when I get ready to make the adjuster, but for the next round of testing I don't want to disturb the spring seat, so that will come later....

Bob
Title: Re: My Benjamin 392
Post by: rsterne on January 06, 2020, 09:31:53 PM
Today was trigger day.... I have been reading about tuning the 392 trigger group, some of the best info was from Timmy@Mac1, who makes the Steroids, of course.... I also read up on the AoA "Super Sear", which has undergone a downgrade, and asked some questions of others who have tuned their own 39X triggers.... I decided to do my own tune, with a bit different "flavour".... Let me make a warning right now about trigger safety.... Many attempts trying to make a conventional trigger into a 2-stage can leave you with an unsafe trigger.... If you don't understand what a "balk fire" is, then you need to read up on it, because it is VERY dangerous.... The short summary is that if you modify a trigger incorrectly, you may create a situation where pulling the trigger through part way (eg. to the beginning of the 2nd stage) can leave a trigger hanging because it doesn't return to full sear engagement.... Imagine a situation where a trigger normally has 1/16" of sear engagement, and without you realizing it, an aborted shot leaves you with the sear engagement only 0.005".... The gun could fire by bumping it.... It you don't know what you are doing, leave your trigger alone.... That means DO NOT copy what I did, because get it just slightly wrong, and your gun may be an accident waiting to happen.... This is a report on what I did, not a recommendation that you follow suit....

The stock 392 trigger is extremely safe, but a very heavy pull weight.... Most of this is because of the geometry of the sear.... When you slowly squeeze the trigger, you are actually moving the hammer back slightly against the hammer spring.... This is most of the reason for the heavy pull weight, but it also guarantees that the trigger will reset and not leave you hanging.... To get around this, I followed the lead of Tim's steroids, and reshaped the sear so that when it is pulled down, it does not move the hammer back.... This makes it much easier to pull, but it won't return if you don't fire the gun and release it.... Tim also grinds the engagement ring on the hammer to match the sear angle, and then rehardens both components for longevity.... I did neither, I'm willing to see how long it lasts, and repair it if it doesn't....

The sear only engages the hammer by the depth of the rear ring on the hammer.... The front ring has nothing to do with the trigger, it is there so that the SHCS on the bottom of the bolt can cock the gun.... There is a second bump on the sear that gives you a "1/2 cock" position if you pull the bolt back a short distance, which causes an audible "click" and leaves the hammer not touching the valve stem.... The angle on the rear sear is the one I worked on.... I changed it from about 17 deg. to about 5 deg.... Basically I made it almost part of an arc of a circle centered on the sear pivot pin.... I was going to make a fancy jig, but got lazy and just did it by hand with a diamond file, checking the angle until I was happy with it, and it didn't move the hammer back when I squeezed the trigger.... I then polished it, and I also polished the top of the tang that the trigger rubs on, and the bottom of the radius on the front part of the trigger that pushes down on the sear tang.... Here is a photo of the sear after I changed the angle and polished it....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Reground%20and%20Polished%20Sear%20Small_zps2zdqt38h.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Reground%20and%20Polished%20Sear%20Small_zps2zdqt38h.jpg.html)

You can clearly see how much I changed the angle.... Now WHAT you are asking yourself is that SHCS doing there?....  ::)

After getting the trigger working the way I wanted, including installing a MUCH lighter coil spring behind the trigger, I found out just how bad the "creep" in the trigger really is.... Since the sear is engaged well over 1/16", the trigger moves back a long ways during the period that sear is moving down on the hammer engagement ring.... I wanted to shorten the engagement, and since the lighter spring behind the hammer provides a "fake" second stage that was very short, I thought long and hard about how to make it "ACT" like a 2-stage by shortening the sear engagement.... The result is much like a worked over QB trigger, which can be quite nice.... It has a "fake" first stage where all that is happening is the trigger is moving against a light spring, and the sear adjustment allows you to set how far the sear moves once the trigger starts to move it.... The problem was, there was nowhere easy to put an adjusting screw in the housing, because it had nothing on the sear to push against to set the sear engagement.... So, I added that screw in the spring hole, and added a 1/8" long #6-32 setscrew in the top of the trigger housing to push against it.... Bingo, I had an adjustable sear engagement, that looks like this (sear shown upside down)....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Adjustable%20Sear%20Engagement%20Small_zpseajpm548.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Adjustable%20Sear%20Engagement%20Small_zpseajpm548.jpg.html)

The setscrew "A" pushes against the head of the SHCS "B", and the further you screw it in, the less the sear engagement is.... Now HOW did I get that 6-32 SHCS into the spring hole in the sear?.... With great difficulty and a little luck....  ::) …. If the sear was not hardened, I could have just tapped the hole, but an HSS tap wouldn't do more than scratch it.... The hole is 0.125", and a #6 screw is only 0.135", so I drilled a 1/16" hole through the screw for the tang on the spring, shortened the screw to a few thou more than the thickness of the sear, put some Loctite 638 on it, and using brute force, wound it into the hole in the sear from the left (under) side, opposite where the hairpin spring is.... I then peened over the end of the screw, and figure it won't come out.... That gave me a "bump" on the underside of the sear for the adjusting screw to push against, so all I had to do was mark where it needed to go, and drill and tap the housing (very close to the edge) for the adjusting screw.... I screwed the adjuster in flush with the top of the trigger group and assembled it for initial testing.... This gives me the minimum sear engagement, and at that adjustment, the assembled trigger looks like this....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Minimum%20Engagement%20Small_zpsqrwjiomp.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Minimum%20Engagement%20Small_zpsqrwjiomp.jpg.html)

I didn't even know if the sear would catch the hammer, but to my surprise it did.... and the trigger felt GREAT !!! ….  8) …. The first "stage" was much longer and very light (because of the light trigger spring), and the force increases significantly when the trigger touches the tang on the sear, because of the fairly heavy (and stock) hairpin sear spring.... I didn't know how much engagement the sear had, and the answer was "not much", as virtually any movement of the second (actual) stage fired the gun.... I tried it several times, and it felt reliable, and I also cocked the gun and banged it around in every direction, thumping it against my workbench and with my fist, and it never fired once.... My conclusion was that the sear, with the adjustment screw flush with the top of the trigger housing was "safe", but with VERY little engagement.... I disassembled the trigger once more, lubed all the contact points with moly paste, and put a drop of blue Loctite on the sear adjusting screw and set it 1/2 turn above flush.... In this position it still misses the bottom of the main tube, and I love the way it feels.... It is nice and crisp, and you can move it a whisker on the 2nd stage pull, but for all intents and purposes it breaks cleanly, with no noticeable creep.... a great hunting trigger, IMO.... Incidently, set this way the "half-cock" does not work.... You can hear it click over the front sear, but with the sear in this position, it won't catch the hammer.... If you back the adjusting screw out all the way, it hits the main tube, but the sear would be fully engaged, just like a stock gun.... It looks like this backed out all the way, so that the head of the SHCS is touching the inside of the trigger housing....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Maxmium%20Engagement%20Small_zpsvansx97y.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Maxmium%20Engagement%20Small_zpsvansx97y.jpg.html)

If you wanted to have the full sear engagement, you could just remove the adjusting screw so it wouldn't hit the tube.... I didn't see how far out I could screw it and still miss the main tube, but maybe 1 turn above flush?.... The point is, I can get a safe sear engagement and still have a nice trigger, so I'm a happy camper.... Remember, do NOT attempt this unless you know what you are doing.... YOU are responsible for your safety and that of others, don't blame me if you have a problem.... YOU HAVE BEEN WARNED !!!!

Bob



Title: Re: My Benjamin 392
Post by: Alan on January 07, 2020, 05:38:57 AM
Very interesting write up. The SuperSear from AoA, is very similar to your sear. As you're no doubt aware, the SuperSear has a longer tang on (its rear end), so it can use the main spring in place of the trigger spring. This is the reason you have to grind out the cast-in boss a bit. I was leery about doing this, so like you, I tried to get the gun to fire using a rubber mallet. It didn't, thankfully!

All I did to the SuperSear was polish all of the "catch" points, as is a stamped-out part, and can be a bit rough on the edges. In real life, I haven't had any issues. There is the same long travel of the trigger before it stiffens up in the last 1/16 of an inch or so, and then lets off very smoothly.
Title: Re: My Benjamin 392
Post by: rsterne on January 07, 2020, 08:35:05 AM
The extended tang on the back of the super sear turns the trigger into a true 2-stage design.... where as you squeeze through the 1st stage you are reducing the sear engagement.... I don't know if it resets reliably if you release it after only pulling through the 1st stage or not (a balk fire), perhaps you have tested that?…. Is your super sear the original, made of hardened steel.... or the new one made of aluminum with just a pressed in  roll-pin on the catchment point?.... That idea might work OK, but just the thought of doing that to save a bit of money on production irked me.... so I decided that was not something I would purchase....

Bob
Title: Re: My Benjamin 392
Post by: rsterne on January 07, 2020, 10:14:47 PM
I decided today to make my velocity adjuster, which may become an SSG.... Here is a photo of the assembled parts....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Adjuster%20Parts%20Small_zpsf6jivhy8.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Adjuster%20Parts%20Small_zpsf6jivhy8.jpg.html)

The body is a piece of 3/4" diameter 6061-T6 aluminum 1.15" long (sized to fit the cavity in the trigger group)…. It is drilled and tapped 3/8"-24 NF for the front 3/4", and the back is drilled out with a Size "W" letter drill, which is 0.386".... That gives clearance for the shank of the adjuster, which is a 1.5" long bolt that has 1" of threads.... I drilled the bolt through with a #11 drill, which is 0.191" to provide a nice sliding fit on a piece of 3/16" drill rod.... There is a piece of it slid through the hole in the bolt in the photos.... If I decide to make an SSG, a longer piece of that drill rod will be the spring guide I will use.... The nice thing about using 3/16" rod is that I can thread it 10-32 on the ends for the front spring seat and adjusting nuts....

You can see a setscrew protruding from the top of the aluminum housing.... It is a #4-40, installed in a tapped hole with Loctite.... The purpose of it is to prevent the round bar from rotating in the trigger housing.... It fits into the slot in the top of the housing.... The next photo shows the adjuster sitting in place in the trigger group....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Preload%20Adjuster%20Small_zps5bf0iurk.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Preload%20Adjuster%20Small_zps5bf0iurk.jpg.html)

I drilled the front of the housing out with the Size W drill, drilling through the original spring seat.... The back of the housing is drilled a bit larger, with a 7/16" drill, as it just needs to provide lots of clearance for the 3/8" adjuster bolt.... You can see another setscrew (a #8-32 that is 1/8" long) in the bottom of the adjuster housing.... It has a short piece of 1/8" plastic rod under it, pressed against the side of the 3/8" adjuster threads to act as a brake.... This allows me to change the position of the adjusting bolt, but prevents it from adjusting itself.... I do most of my adjusters that way....

The way the adjuster is currently set has the front of the adjuster bolt in the same location as the original spring seat.... so the gun will behave as if the adjuster wasn't there.... The 1.5" long bolt can move forward about another 1/16" to increase the hammer spring tension, and has more than 1/2" of adjustment to the rear, for a total of about 5/8".... If I need a more forward position for the spring seat, I can add a spacer/spring guide to provide that.... but in reality I will probably need to move the adjuster back, to allow for the installation of an SSG at a later date.... The ability to adjust the hammer strike will greatly enhance the ability to tune the gun as a retained air pumper (RAP, aka ACP - air conserving pumper)….

I assume that existing ACPs, such as that sold by AoA, must have some way of tuning the hammer strike.... but if so, I don't know what they use.... I think that this version will work great....

Bob
Title: Re: My Benjamin 392
Post by: Alan on January 08, 2020, 06:34:52 AM
One thing is for sure, it is off-season for tourists!
Title: Re: My Benjamin 392
Post by: rsterne on January 08, 2020, 07:32:36 PM
Today I drilled out the barrel port, slimmed down the bolt probe, and drilled the holes in the tube for the side valve screws.... It turned out to be a LOT of hand work, mostly because I could not grasp the soldered barrel and tube assembly in my very small milling attachment on my lathe.... The side holes for the valve I had to drill undersize in approximately the right place, install the valve with the bottom screw, mark the position of the side screws, and then Dremel the holes over so that they fit the screw heads nice and snug.... It took a long time, but the results are great.... When I install the side screws, they are snug in the holes in the tube, and the bottom valve/trigger screw threads in with NO resistance, it lines up perfectly.... I am a LOT happier with the strength of the valve mounting now....  8)

Drilling out the barrel port was likewise problematic, because I could not mount it in the lathe milling attachment.... I drilled it out one number drill size at a time until I got to 11/64" (0.172"), which is 79% of the bore.... Since the exhaust hole in the valve measures 3/16" at the top, because it was milled on an angle, I used a 3/16" center drill to put a slight taper in the barrel port, so that there is no step at the valve, but the port at the boreline stays at 0.172"....

Even the bolt probe was a problem, because with the bolt handle attached (it appears to be one piece) I could not chuck it in my lathe.... I ended up removing the O-ring, and sanding the probe smaller on the edge of a disc sander, and at the same time tapered the front of the O-ring gland, because it partially restricts the (now larger) barrel port.... Here is a photo of the thinned out bolt probe and the side valve screws (which are just above the stock)…. At the back of the tube you can see the hex head on the bolt for adjusting the hammer spring preload.... At the moment it is set for stock preload....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Valve%20Screws%20Adjuster%20and%20Bolb%20Probe%20Small_zpsm4mxoo8s.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Valve%20Screws%20Adjuster%20and%20Bolb%20Probe%20Small_zpsm4mxoo8s.jpg.html)

While I was working on the tube, I investigated why the valve was so tight when in position.... I had to drive it back the last inch, and it was almost impossible to turn to get the screws lined up.... It turns out the the roll-stamp on the left side is so deep that it dented the tube on the inside.... I had to file and sand off the bump on the inside (not an easy job), but it turned out really well, and now the valve slides in to position the way it should....  ::)

Here is a photo of the new front pins.... They have a single nut peened and loctited in place on one side, and double nuts on the other.... I will eventually get a couple of Ny-Loc nuts instead, they will be simpler and look better....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Front%20Bolts%20Small_zpsyuf2jvy7.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Front%20Bolts%20Small_zpsyuf2jvy7.jpg.html)

The last photo shows where the pump handle sits with no pressure in the air tube.... I adjusted the gap in the valve to 0.050" to achieve this position.... It is easy to remember, because the top of the pump handle is level with the bottom of the stock....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Pump%20Position%20Small_zpsuieox1sn.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Pump%20Position%20Small_zpsuieox1sn.jpg.html)

These changes took the whole afternoon.... After dinner I did some testing, the results will be in the next post....

Bob
Title: Re: My Benjamin 392
Post by: rsterne on January 08, 2020, 07:51:28 PM
OK, time to test this beast.... All the port mods are now done, both in the valve and barrel, including a slimmer poppet stem (compliments of the MRod poppet), larger valve throat and exhaust port, and a slimmer bolt probe.... The smallest port in the system is the barrel port, which is 0.172" (79% of bore)…. It was 0.154" stock, so I have increased the area by 25%.... Everything else in the system can flow more air than that.... I had the hammer spring preload adjuster set so that the preload was exactly the same as stock.... The gun does not retain any air even at 14 pumps (more about that later)…. I checked the velocity every 2 pumps, ending at 14, and here is the results....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Benjamin%20392%20Velocity%20Port%20Mods_zpsbjnqpa07.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Benjamin%20392%20Velocity%20Port%20Mods_zpsbjnqpa07.jpg.html)

The dotted lines are the previous results, with the valve mods done, but not the barrel port and bolt probe.... Obviously they were hampering the airflow.... The larger ports make a big difference at higher pump numbers, as expected.... I then tried different pellet weights at 8 pumps and 12 pumps.... Here are those results....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Benjamin%20392%20Weights%20Port%20Mods_zpskzssvvpz.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Benjamin%20392%20Weights%20Port%20Mods_zpskzssvvpz.jpg.html)

The solid lines are at 8 pumps, and the dotted lines are at 12 pumps.... The gun now exceeds 22 FPE with the 25.4 gr. Monsters at 12 pumps....  8) …. I did one additional test, using my 27.4 gr. BBT HPs at 14 pumps.... I got 631 fps, which is 24.2 FPE.... I'm pretty happy with that, still using the stock pump....

I then backed off the hammer spring preload, because I was curious when the gun would start retaining air.... I backed it off a full 1/2" from stock, and it still showed NO velocity loss at 8 pumps.... but it was a LOT quieter.... I then tried 10 pumps, still no retained air.... At 12 pumps, it finally retained a small puff of air.... It looks like I can make an SSG, probably using the stock spring, and still have it dump 14 pumps.... That may be my next project....  ;)

Bob
Title: Re: My Benjamin 392
Post by: Insanity on January 09, 2020, 11:04:04 AM
Sorry if I missed it but what is the purpose of having that gap in the pump handle?
Title: Re: My Benjamin 392
Post by: Alan on January 09, 2020, 12:12:39 PM
I believe it wasn't "fully" closed.

This whole exercise that Bob is still going through (by all accounts) is a very special case. Very few airgun fanatics would even attempt to do what he does, and has done! This is one of the reasons Bob won the first-annual, Airgun Guild Award. His prize was a Labradar with all of the trimmings.

Nonetheless, your question is valid. Bob will answer!
Title: Re: My Benjamin 392
Post by: rsterne on January 09, 2020, 02:12:35 PM
That last bit of pump travel, which is actually only a few thousandths of an inch at the end of the piston, compresses the rubber cup on the end of the piston.... Since it is not a perfect fit against the end of the valve, that squeezes out a bit of air in the "headspace" of the valve.... Dead volume between the piston and valve, called headspace, is air than cannot enter the valve, so acts as part of the valve volume.... Having the pump cup touch the valve before the arm is completely closed helps minimize the headspace when you do close it all the way, which you do on every stroke.... It takes very little force to move it that last bit, because the linkage rocks over center, and the leverage is maybe 100:1 for that last bit of motion....

Headspace has two effects.... It decreases the pump efficiency (since you are compressing air that you don't end up using)…. and ultimately it limits the pressure that you can get in the valve.... Having an easy to remember the location of the handle (eg. flush with the stock) tells you if you got it right during a rebuild.... and also lets you know if anything is wearing, as the distance would decrease....

Bob
Title: Re: My Benjamin 392
Post by: Insanity on January 09, 2020, 02:39:40 PM
Thanks for the explanation kinda reminds me of having to set headspace and timing on a M2. If you get it wrong it dont work well.
Title: Re: My Benjamin 392
Post by: rsterne on January 09, 2020, 06:51:13 PM
This afternoon I made and installed an SSG (stopping spring guide).... Here is a photo of it, ready for installation....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/392%20%20SSG%20Small_zpssjrvr1nd.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/392%20%20SSG%20Small_zpssjrvr1nd.jpg.html)

The concept is quite simple.... There is a 3/16" spring guide rod 4" long that slides through the adjusting bolt.... On the front is a nut threaded and glued in place, and then turned down to fit inside the hammer.... At the back the rod is threaded 10-32 and a pair of nuts are used to adjust the preload on the spring.... In this case, I have 0.50" of preload, which is about 3.5 lbs.... There is a small O-ring on the rod between the rear nuts and the head of the adjusting bolt, to absorb the impact when the spring guide stops, and reduce the noise of that collision.... The entire assembly threads into the aluminum mounting sleeve in the top of the trigger group.... I chose 3/8"-24NF threads so that the assembly may be removed to change the preload or spring without disassembling the gun.... Here is a photo of it installed....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/SSG%20Installed%20Small_zps1lg2pd2b.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/SSG%20Installed%20Small_zps1lg2pd2b.jpg.html)

Turning the hex head bolt in towards the gun reduces the gap between the end of the spring guide and the bottom of the hole in the back of the hammer.... The idea is to have a small gap between them, so that the spring stops pushing the hammer just before it hits the valve stem.... The hammer then carries on from its own momentum, strikes the stem and opens the valve.... The hammer is NOT preloaded by the SSG, it can "rattle around" in the gap between the valve stem and the spring guide....

The magic happens when the valve closes after firing.... The hammer is thrown back against the now stationary spring guide, but because of the preload on the spring, and the mass of the guide rod, instead of recompressing the spring and getting thrown back at the valve and reopening it (the infamous hammer bounce)…. it just rattles around, without enough energy to open the valve a second (or third) time and wasting air.... Changing the gap adjusts how hard the hammer hits the valve stem, because as you increase the gap, you reduce the amount the spring is compressed when you cock the gun.... I tested the gun with the SSG in place, and I got a pleasant surprise....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Benjamin%20392%20Velocity%20SSG_zpswdr6i8vx.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Benjamin%20392%20Velocity%20SSG_zpswdr6i8vx.jpg.html)

The solid blue line is the velocity with 15.9 gr. pellets as I increased the SSG gap.... As you can see, the velocity did not begin to drop off until I had 6 turns of gap (1/4")…. When I compared the velocity with what I got yesterday, without the SSG, I was surprised to see that with no gap (and indeed yesterday without the SSG) I got 659 fps, which increased to 666 fps as the gap was increased to 3 turns (1/8") and over the next 2 turns it returned to 659 fps.... I have no concrete reason why the velocity should increase about 1% with the SSG, but I suspect that without the SSG, the hammer is bouncing off the back of the valve, and that is actually reducing the dwell a whisker.... Anyway, the important thing is that I can have 5 turns of gap and not lose any velocity with the SSG installed....

Now remember that without the SSG the gun did not retain any air, all the way to at least 14 pumps.... Now it retains a puff of air at 8 pumps with the SSG installed, even at zero gap.... At 5 turns of gap, it is retaining enough air to produce a 2nd shot of over 400 fps (the dashed blue line)…. That is roughly the velocity the gun shoots at 3 pumps.... so in theory I could pump 5 times and get back to the pressure I had without the SSG with 8 pumps.... That is why they call this type of gun a "Retained Air Pumper" (RAP, aka Air Conserving Pumper or ACP)…. 8)

As I continued to increase the SSG gap, the velocity of the first shot dropped, and that of the second shot increased.... By the time I got to 9 turns of gap (3/8") the second shot was slightly faster than the first.... Not only that, but there was enough air left to produce a 3rd shot, with the gap set to 7 turns or more (dotted blue line)…. I repeated the testing at 12 pumps, the results are in red above....

If I adjust the SSG gap to about 8.5 turns, I should be able to tune the gun to have two equal shots of about 550 fps (10.7 FPE) without pumping in between.... However, since there is a bit of air left after the second shot, I can't just pump 8 pumps to repeat the procedure.... However, I might be able to pump it about 6 times after every 2 shots, and keep getting a string of pairs of ~11 FPE shots, for only 3 pumps per shot.... instead of 5 pumps per shot to get the same power without the RAP design and the SSG....

The SSG opens up a whole range of possibilities of different ways to tune my 392.... With 2 turns of gap, I could pump to 12 pumps, and get a shot of 755 fps (20 FPE)…. There would be enough air for a 2nd shot of about 550 fps (normally about 5 pumps), so I should be able to add 7 pumps (instead of 12), and keep shooting at 750 fps for as long as I want to pump.... The best way to do an RAP is to have a gauge.... That lets you know exactly how many pumps you need to get back to the pressure you need for the power you want.... The really cool part is that you have fewer pumps required to get back to full power....

If you need any further proof that an SSG really changes things, just listen to the difference.... Without it, you can hear the hammer bounce, with that telltale burrrppppp sound.... With the SSG, you just hear one POP.... and as you tune it to lower power levels it gets a LOT quieter....  8)

Bob
Title: Re: My Benjamin 392
Post by: steveoh on January 09, 2020, 07:16:24 PM
I love reading your posts! Kinda makes me want to buy a pumper. Kinda .
Title: Re: My Benjamin 392
Post by: rsterne on January 10, 2020, 09:43:25 PM
I pulled the 392 apart again today and measured it up for a new pump piston with a flat face and an O-ring seal.... and decided that would also need a new valve front end and check valve assembly.... The only thing that really makes sense is to make that front end as a 2-piece valve, from a time point of view.... That part of the project will wait until I get some pressure measurements with the existing valve and pump to provide a baseline....

I measured up the valve and cut two new O-ring grooves, one on the back half, just in front of the existing exhaust port and valve screws, and the other on the front half, far enough back to allow a gauge between it and the front O-ring.... I used a # 113 O-ring for the front one, and a # 016 for the back one.... The rear groove is just behind the valve seat (where the valve is solid).... and the front groove is just ahead of the shoulder where the washer sits between the check valve spring and the valve spring (where the valve wall is the thickest).... There is plenty of room between the front two O-rings for a gauge port and two side mounting screws, which will be necessary when I make the 2-piece valve.... Here is a photo of the valve with the distance between the O-ring grooves dimensioned....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/O-ring%20Dimensions%20Small_zpsaxeqaf6o.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/O-ring%20Dimensions%20Small_zpsaxeqaf6o.jpg.html)

I clamped the main tube in my milling attachment on my lathe, centered off the bottom valve screw hole, and drilled a gauge hole 2.25" ahead of that.... Since I am using a 1/16" to 1/8" NPT adapter for the gauge, the hole in the tube is 5/16" which will just clear the adapter threads.... I installed the valve, with the two halves unscrewed with the same 0.050" gap as before, and scribed the gauge location between the front O-rings.... That looks after not only the location, but the clocking of the threads (at the gap selected)…. I then drilled and tapped the valve at the marked location to 1/16"-27 NPT.... The hole is in the check valve section so there is lots of thread depth.... I cleaned and reassembled the valve, and reinstalled it in the gun, and threaded in the gauge and adapter.... It looks like this....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Gauge%20Installed%20Small_zpso7codhv8.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Gauge%20Installed%20Small_zpso7codhv8.jpg.html)


I laid out and drilled a 1" hole through the stock with a hole saw, and then using a Dremel sanding drum fitted it to a fairly close fit around the gauge.... Once the gun is assembled, the gauge sits in a hole in the stock much like on a Disco....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Gauge%20in%20Stock%20Small_zpsvsj5jfkm.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Gauge%20in%20Stock%20Small_zpsvsj5jfkm.jpg.html)

I'm quite pleased with the appearance.... The gauge I had was a 3K gauge, and I should probably get one that has a 2000 psi scale.... However, it will at least let me find out what pressures I am pumping to now.... I don't know how accurate the gauge is, but it doesn't really matter, as all I want is to be able to pump to the same pressure for each shot, and to compare changes to the pump and when I change the valve.... It looks like 12 pumps with the stock pump, adjusted the way it is, and stock valve volume, is about 1200 psi.... The valve volume is slightly larger because of the gauge and adapter, maybe 10%.... It doesn't leak down while taking a couple of shots to test it, so I have put 5 pumps in it (just over 500 psi) to leave it overnight to see if it has a slow leak....

Bob




Title: Re: My Benjamin 392
Post by: rsterne on January 11, 2020, 12:06:47 PM
Well, the gun didn't leak at all overnight....  8) …. However, I am getting a puff of air out the side screw holes on firing, particularly at high pump numbers....  :(

Today I tested the velocity and pressure at every pump stroke up to 15 pumps.... That was just over 1500 psi, which I am going to use as a self-imposed limit (at least for now) because of using the stock pump linkage, and because I want to maintain the safety margin on all components, particularly the valve screw pockets in the tube.... I did the same on my .25 cal Disco Carbine.... I pumped it to 1800 psi to test it, and then backed off to 1500, for longevity, and also because it was harder to pump than I liked at 1800....  ::)

I calculated the additional volume from adding the gauge, which was about 0.4 cc, so the total volume has now increased to about 4.6 cc.... This means that for any given number of pump strokes the velocity and FPE have dropped a few fps because the pressure has dropped by about 10%.... This is because the swept volume is the same, but since the valve is larger the pressure gain on each pump stroke is a bit less.... In other words, it takes about 11 pumps now to get the same pressure as I had before at 10 strokes.... However, if you take that into consideration, the gun is developing more power at the same pressure than before, because of the extra valve volume, which causes a lower pressure drop during the shot.... I set the SSG at 1 turn of gap for this testing, and recorded the pressure and velocity for the first shot, and once the gun began retaining air, I did the same for the second shot (dotted lines)….

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Benjamin%20392%20Velocity%20Presure_zpstgfhimpl.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Benjamin%20392%20Velocity%20Presure_zpstgfhimpl.jpg.html)

This gives me a really good baseline for further work.... As I increase the valve volume, the pressure gauge is a necessity to know how many pumps to use to get back to the same pressure.... It will be equally important when I start to work on the pump, not only to measure any gains in pump efficiency, but to make sure I am not overstressing the components.... When I compare this pressure curve to the graphs in Reply #66, the closest match is a headspace of 0.3 cc (because I adjusted the valve length a bit longer) and an efficiency of about 75% (when corrected for the increased valve volume)…. The next step is to remove the valve and bore it out to increase the volume....

Bob
Title: Re: My Benjamin 392
Post by: calinb on January 11, 2020, 01:29:23 PM
I have never had a Benjamin 392 pumper before, and Eric at Scopes and Ammo was clearing them out last fall, so I had my wife pick one up for me for Christmas, and I opened it today....<snip>
Bob, I first read your post on the other forum and then came here (where my post is less likely to get pulled, I think). I still need to study the rest of this thread but gotta go plow some snow right now. This thread looks like it's picking up more steam than the other one too. Good stuff here!

I have both a 392PA and 397PA and one of the last Racine Silver Streaks made, which I had Tim McMurray Steroid for me. The old Yellow and Green forums had a treasure trove of info, but between their decommissioning and the loss of countless photobucket pics, I can't find most of it any longer. (The tapatalk archives seem to be almost unusable to me.)

In a nutshell, there are two different directions to take these guns: Maximize air dump and power (Tim's approach with the Steroid mods) or take the air conserving pumper (ACP) mods approach (as promoted by Steve Woodward on the old Yellow) for multiple shots on an air charge. Tim and Steve argued a lot online. Tim posted a ton of info about pumpers and his Steroid performance mods on GTA (actually shared with everyone pretty much what he does). Hopefully it's still all in the GTA somewhere, but ya' never know on any airgun forum, with all the fighting and banning that goes on!

https://www.airgunsofarizona.com/BenjaminACP.htm (https://www.airgunsofarizona.com/BenjaminACP.htm)
http://mac1airgun.com/steroidbenjaminsstreaks.html (http://mac1airgun.com/steroidbenjaminsstreaks.html)

Tim did my Streak Steroid, which is the only way to get an official "Steroid" (and also did a beautiful job silver soldering the barrel under the rear sight--a common separation point) but I also have a bunch of Tim's spare parts and a bunch of ACP mod parts just sitting in a box. I'll probably "Steroid" my 392PA eventually but I might install some ACP parts in my 397, including a small CO2 cylinder as an air tank under the pump fore end.

The newer 392PAs tend not to be very accurate. Tim warns to not spend money on them or extend their power and longevity with mods (Steroid), if they are not accurate and thus not worthy. I've tested a couple dozen types of pellets in my 392PA and the JSB Jumbo Heavy is the only accurate pellet that I've found; my 392 is very pellet picky (maybe due to its very shallow rifling, which is common in these guns). I've slugged all my barrels. BTW, I once spent a few hours searching the web and compiling a spreadsheet of online accuracy reports on these rifles (dozens of online reports). I simply indiscriminately put the data into the sheet and found the 392 reports to be significantly lower in accuracy, on the average. The 397s accuracy reports were the most favorable and Sheridans were close. If you can't get U.S. quarter size 10-shot groups at 25 yards (center-to-center measurement), try JSB Jumbo Heavy pellets!

My 397PA is just the opposite--a tack driver with just about any pellet. My Silver Streak is also accurate but there are not many 20 cal. pellets available to test (still need to test the brand new JSB 20 cal. Heavy pellet). I use the 13 gr. JSBs or sometimes H&N Hunters in it.

I think I have a bunch of large Internet forum and document archives about the "Bens" and "Dans" that I downloaded somewhere. If you browse through them, you should get some ideas. I'll PM you, if I can find them on my disk drives. I studied the subject for countless hours online several years ago but never found the time to benefit from my learnings in project completions. :(

BTW, Most people just install an adjustable piston to remove excessive headspace. You can mod your stock piston or buy one from Tim (but I know you'll mod your own).

http://www.mac1airgunshop.com/sheridan-rocker-bolt-lug-p/m1stadpis.htm (http://www.mac1airgunshop.com/sheridan-rocker-bolt-lug-p/m1stadpis.htm)

-Cal

Title: Re: My Benjamin 392
Post by: rsterne on January 11, 2020, 04:19:29 PM
Hey, Cal.... thanks for the summary, much appreciated.... I wasn't sure which way I was going when I started this project, but it looks like I'm going to end up with a Retained Air Steroid with a twist of Canuck....  ::) …. I will be making my own adjustable flat-topped piston, like I did for my Millennium Pumper once I exhaust the possibilities with the stock pump....

Today's project was to bore out the valve, removing as much metal as I could safely, without cutting holes in the mid section....  Even though I currently have the two extra O-rings needed to do that, I wanted to see how much I could increase the volume and still have the valve look stock on the outside (except for the gauge port)…. At this point, the two extra O-ring grooves are not needed, even for the gauge, since it seals to the valve with Teflon tape.... Here is a photo of the outside of the valve at this point....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Valve%20with%20Gauge%20Small_zpsinuvqvw7.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Valve%20with%20Gauge%20Small_zpsinuvqvw7.jpg.html)

The gauge port is exactly 2.25" ahead of the lower valve screw hole, measured center-to-center.... The  first thing I did on the valve to increase the volume was to cut off some of the threads.... I left the stub 3/8" long, measured from the shoulder…. I then bored out the inside of the valve front to 5/8" ID, from about 1/16" ahead of the shoulder where the O-ring sits down to the shoulder where I had the spring seat.... The length of that bored out section ended up at 5/8".... I had to make a new double ended spring seat to hold the check valve spring and the front end of the valve spring, I just turned it out of aluminum.... It is 3/8" OD on the front end, 0.450" in the middle, and drilled to 0.29" to accept the outside of the much lighter check valve spring.... I also shortened the check valve some more, to make up for the extra volume of the new seat.... The back of the seat is 1/4" OD to fit inside the valve spring, and the through hole is 3/16".... Here is a photo of the "guts" of the valve, ready to install....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Poppet%20Assembly%20Small_zpsofxnkfkz.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Poppet%20Assembly%20Small_zpsofxnkfkz.jpg.html)

The back half of the valve also got bored out to 5/8" ID, which just removed the threads inboard of what I needed to secure the two halves together.... The 5/8" ID carried on all the way back to the solid area where the seat is.... That section of the 5/8" ID is about 3/8" long, starting from 5/16" inside the valve, leaving the same threaded length as on the stub, once you allow for the 0.050" gap when assembled.... Here is a photo looking inside the valve halves, to show where they were hogged out....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Bored%20Valve%20Small_zpsvgzaqu26.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Bored%20Valve%20Small_zpsvgzaqu26.jpg.html)

I calculated the increase in volume from the machining, and it works out to 2.1 cc.... That would be a 50% increase on a stock valve, but with my gauge port and adapter, my total volume is now 6.7 cc.... That is 50% larger than with the gauge installed, and a 60% increase over stock.... To get to the same pressure, it should therefore need 50% more pumps than the graph I posted in my last post.... We'll soon see, as the next thing to do is reassemble the gun and repeat the last set of tests....

Bob

Title: Re: My Benjamin 392
Post by: rsterne on January 11, 2020, 06:51:35 PM
After dinner I ran the same tests I did with the stock valve with gauge.... I measured the pressure and velocity of the first and second shot every 2 pumps up to 20, where the pressure exceeded 1500 psi.... I was still using the same 15.9 gr. pellets as all the other similar tests.... As expected, I require more pumps to get to the same pressure as before, because the valve volume is 50% larger, at 6.7 cc.... I am still running the SSG at 1 turn of gap.... Here are the results....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Benjamin%20392%20Vel%20Press%20Bored_zpst375pqx4.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Benjamin%20392%20Vel%20Press%20Bored_zpst375pqx4.jpg.html)

It looks like I have gained significantly in pumping efficiency.... With a 50% valve volume increase I had expected 50% more pumps to get to the same pressure, but these results are significantly better than that.... I got 1520 psi with 20 pumps, whereas I expected that to take 22 pumps.... This 2 pump advantage over what I expected occurred all the way along the pressure range, from 600 psi and up.... The first few pumps are MUCH easier, which I think is from increased pump efficiency from filling the larger valve.... There is probably also an efficiency gain because the headspace, which hasn't changed, is a smaller percentage of the larger valve volume....

The velocity and energy is significantly greater at the same pressure with the larger valve volume.... This is much like what happens when you install a larger plenum in a PCP.... In addition, the residual pressure for the second shot is also greater, and should increase even more when I dial back the SSG.... At 1520 psi (20 pumps) I got 827 fps with the 15.9 gr. pellets (24.2 FPE), and the 2nd shot was 622 fps (13.7 FPE)…. for a total of nearly 38 FPE.... I tried the 25.4 gr. Monsters at the same pressure, and hit 698 fps (27.5 FPE) with a 2nd shot of 524 fps (15.5 FPE) for a total of 43 FPE.... The next thing to test is to see what happens to the velocity of both shots when I dial the hammer strike back by increasing the SSG gap.... It will be interesting to see how many pumps per shot are required to get a continuous string of 20 FPE shots....

Bob
Title: Re: My Benjamin 392
Post by: Alan on January 12, 2020, 05:16:33 AM
Geez! Twice the FPE! A lot more pumps to be sure, but it does show what could (can) be done if one is willing. All of this makes me wonder....

Benjamin advertises their 397 as developing (up to) 14.9 FPE. Maybe they pumped up their test model with Helium, because as close as I could get, was just over 10. Even after I "slightly" modified the valve, I only got a bit over 12 FPE. Makes me wonder what a few factory mods could do?
Title: Re: My Benjamin 392
Post by: calinb on January 12, 2020, 01:01:40 PM
Anyways, on reassembly with the aluminum valve with the 2-spring mod. I found out that the hammer spring was holding the valve open, so I couldn't pump it.... I think this was causing the hammer to jam on firing (the hammer moving too far forward and catching on the forward bump on the sear)…. so I could not proceed further.... I pulled the gun apart again (cursing those roll-pins again, I really have to change those out)…. and disassembled the valve and found another valve spring, in between stock and the one I tried.... Here are the 3 springs....
I'm not sure that I understand all of the above, Bob. It's common for the hammer to hold the valve open with modified guns (Steroid instructions say to cock the gun before pumping and, as I recall, older Sheridan manuals may have said the same thing).

What was causing the hammer to jam on firing and how does it relate to getting the first pumps into the gun?

Are you pulling the valve out through the muzzle end each time you disassemble? You should pull it out from the breech end and then there's hardly ever a reason to remove the roll pins (only for work on the pump piston section). My 392 required a lot of work inside the tube to get the valve out easily though. The problem is Crosman stamped the lettering too deep on the outside of my tube and distorted the tube. I managed to get it out eventually (even without resorting to punching it back from the muzzle end, which would've required removing the roll pins, of course). It's standard technique to use the pump pressure to help to drive it backwards--just be careful of the valve becoming a projectile! I made a mandrel to fit inside the tube and worked the lettering bumps mostly out, but I also had to use a long sander and take off a small amount of brass inside the tube too, followed by polishing. It was a PITA but valves SHOULD come out the breech end!

I'm slowly working my way through this thread. More later when I find more time today but I wanted to mention this before you pull your gun apart from the front too many more times!

Edit: You actually don't have to fully cock the gun. Just hold the bolt back a bit to get a pump or two into the gun. Then the pressure will hold the valve closed against the hammer pressure. I usually just fully cock and then de-cock.
Title: Re: My Benjamin 392
Post by: calinb on January 12, 2020, 04:01:27 PM
The other item is a tool to rotate the valve when installing it so that the screw hole (and therefore the transfer port) line up properly....
Clever tool, Bob! I've never needed it, because I can usually keep the holes lined up while I push the valve into place the short distance from the breech, rather than having to push it in nearly the full length of the pump tube from the muzzle end. If I miss alignment a bit, I just give the pump arm a quick partial pump, push the valve back out and try again. I usually get it the first time though.

It would be hard to come up with something that hasn't been done before on a 392....

For sure!

So, I added that screw in the spring hole, and added a 1/8" long #6-32 setscrew in the top of the trigger housing to push against it.... Bingo, I had an adjustable sear engagement, that looks like this (sear shown upside down)...

But you've probably done it, Bob. I've never seen this adjuster approach before now. Very nice! I've seen adjusters installed to push near the top of the bolt safety extension arm of the trigger (extension arm only slightly shortened to enable de-cocking, but not completely removed).

The extended tang on the back of the super sear turns the trigger into a true 2-stage design.... where as you squeeze through the 1st stage you are reducing the sear engagement.... I don't know if it resets reliably if you release it after only pulling through the 1st stage or not (a balk fire), perhaps you have tested that?…. Is your super sear the original, made of hardened steel.... or the new one made of aluminum with just a pressed in  roll-pin on the catchment point?.... That idea might work OK, but just the thought of doing that to save a bit of money on production irked me.... so I decided that was not something I would purchase....
I installed the original SS into my 397PA. It resets reliably. I installed one of the newer aluminum versions on a friend's gun. It does not reset reliably and I never solved the problem. SS designer Steve Woodward told me that it should reset, but it does not. I cautioned my friend to always recock after a balked trigger pull, as is necessary with many airgun triggers.

I ground my 392PA sear to realize a bit of two stage action, along these lines:
https://www.airgunsmith.com/392/triggertune.htm (https://www.airgunsmith.com/392/triggertune.htm)
I also slightly re-profiled the striker ("hammer") engagement surface of the sear to reduce its "positive" engagement against the striker. I still have a bunch of Kasenit so I surface hardened the sear after these grinds. A muzzle loader smith told me there's nothing available that's as good as Kasenit today. I repositioned the weaker hardware store trigger return spring ABOVE the trigger pivot point to remove all the trigger slack and slightly help my finger to oppose the strong hairpin spring.

My third trigger in my Steroid Silver Streak has Tim's treatment. All three of my gun's triggers work well.
I assume that existing ACPs, such as that sold by AoA, must have some way of tuning the hammer strike.... but if so, I don't know what they use.... I think that this version will work great....
Steve Woodward has designed and documented fully adjustable hammers, but his ACP sold by AoA only offers two power settings via two cocking positions on a version of his SuperSear. This Dual Power sear is only available on the AoA gun, AFAIK.
Title: Re: My Benjamin 392
Post by: rsterne on January 12, 2020, 04:26:37 PM
Hey, Cal.... I'm so used to taking the valve out through the front (13XX with that raised lug to prevent the valve moving back, and Discos where the slot in the back carves up the O-rings) I never thought of pulling it out the back.... ::) …. Yeah, that roll-stamp really messed up my tube, I had to file/sand it away.... now I could probably take it out the back but old habits die hard....  ;)

When you get further into the thread, you will see that I'm running an SSG, so the hammer has a gap to the valve stem all the time (even uncocked)…. I could actually use a much lighter valve spring now, but the MRod spring is working fine and I can't be bothered changing it.... The required hammer strike with the smaller MRod poppet with the 1/8" stem is so low the gun is easy to cock, no need to change.... If you don't know about SSG's, stand by, this post will rock your world....  :o

OK, guys, now we're into the meat and potatoes of this build....  ;D …. Today I wore out my arm completely, shooting out all the air in the valve at 4 different pressures and up to 15 different SSG gap settings.... Here are the results, on 2 charts so they aren't so crowded.... In all cases the solid line is the first shot, the dashed line the second shot, and the dotted line is the third shot.... By the time the 3rd shot got to about 500 fps there was a 4th shot, and although I recorded it, I didn't graph it, it would have made the graphs even harder to understand....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Benjamin%20392%20Retained%20Air%206-10_zpsf5z3nyol.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Benjamin%20392%20Retained%20Air%206-10_zpsf5z3nyol.jpg.html)

On the above chart, the red lines are at 6 pumps (about 600 psi), and the blue lines are at 10 pumps (about 900 psi)…. You will note that the plateau (665-670 fps) on the 10 pump curve extends to 8 turns of gap (over 5/16") and on the 6 pump curve (550-555 fps) past 10 turns of gap (about 7/16")…. Reducing the gap for more consistency by using a lighter spring and/or less preload would be better.... MAN is the gun easy to cock with the SSG installed, and would be even nicer with a lighter spring and smaller gap.... Now the details for these pressures....

At 6 pumps, all three curves cross at 14 turns of gap, giving a 3-shot bell curve of 385, 400, 360 fps (a 10% ES)…. This is only 5.1 FPE average, and I didn't explore it further.... At 10 pumps, I can get two shots of 555 fps (10.9 FPE) without repumping, at 10.7 turns of gap.... It takes 7 turns to repump back to the same pressure as I get at 10 from empty, or if I only shoot 1 shot, just 3 pumps.... and I can keep shooting at 550-555 fps with 3 pumps required per shot.... That actually works out to 1.02 FPE/CI, based on the volume of air pumped (58 cc or 3.54 CI per stroke)…. which is very good for a pumper....  8)

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Benjamin%20392%20Retained%20Air%2014-20_zpsamw0pafe.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Benjamin%20392%20Retained%20Air%2014-20_zpsamw0pafe.jpg.html)

On the second chart, the red lines are at 14 pumps (about 1200 psi), and the blue lines are at 20 pumps (just over 1500 psi)…. The plateau at 14 pumps (742-748 fps) extends past 6 turns of gap (1/4"), and at 20 turns the velocity starts to drop from 824 fps after 3 turns of gap (1/8")…. With the SSG set at just over 3 turns of gap, I still have maximum power available, but over half the air is retained, so I only have to pump 10 pumps to get back to 24 FPE, instead of 20 pumps.... The same thing applies at the 14 pump level, set for where the velocity just starts to decrease, it only takes 7 pumps to refill to 19.5 FPE, instead of 14 pumps.... For maximum power work that means the hammer spring and preload are OK, but I could still probably reduce the preload, or use a lighter spring, particularly for a 2-shot tune at 14 pumps.... 

At 14 pumps the first and second shots have the same velocity (640 fps) with the SSG set to 8.5 turns of gap.... That gives two shots at 14.5 FPE without repumping between, and takes 10 pumps (instead of 14) to repeat those 2 shots.... If I choose to fire a single shot, I have to alternate 4 pumps and 5 pumps to maintain the velocity as close as possible to that 640 fps mark.... Looking at the curves for 20 pumps, I can get two shots of 725 fps (18.6 FPE) by setting the SSG gap to 5.7 turns.... It takes 14 pumps to get back to just over 1500 psi, or I can shoot single shots and pump 7 times each.... Here is a short string, showing the velocity and the number of pumps in between, mixing 2-shots and single shots between pumping....

Pump 20 times
2 shots, 732 & 726 fps
14 pumps
738 fps
7 pumps
721 fps
7 pumps
727 fps
7 pumps
2 shots, 723 & 722 fps

The gauge makes this easy, I just pump until the pressure is just over 1500 psi, and I can take 1 or 2 shots, and stay within less than a 3% ES....  8)

My conclusion is that this bored out valve, with a volume of 6.7 cc, is too small to be able to get a useful 3-shot bell curve without pumping.... By increasing the volume by using a 2-piece valve, that could probably be accomplished, at the expense of a lot more pumps for the initial fill.... However, it is entirely possible to get 2 equal shots at any power level desired from 10 FPE up to probably 20 FPE (by using heavier pellets), and be able to shoot a mix of single or two shots by repumping to the appropriate pressure level.... It requires roughly 1/3 as many pumps as the initial fill for each shot fired when tuned for 2 equal shots.... If tuned for full power, the SSG allows enough retained air to cut the number of pumps require to continue shooting at full power in half....  :o

The gun set up the way it is right now is pretty close to accomplishing my original goal.... I can get 2 shots within a 2% ES at 19 FPE (688 fps with the 18.1 gr.) at 20 pumps (1500 psi), with 7 pumps per shot required to refill.... I was hoping I could get 2 shots at 20 FPE....  ::)

Bob
Title: Re: My Benjamin 392
Post by: calinb on January 12, 2020, 09:37:57 PM
Hey, Cal.... I'm so used to taking the valve out through the front (13XX with that raised lug to prevent the valve moving back, and Discos where the slot in the back carves up the O-rings) I never thought of pulling it out the back.... ::) …. Yeah, that roll-stamp really messed up my tube, I had to file/sand it away.... now I could probably take it out the back but old habits die hard....  ;)

When you get further into the thread, you will see that I'm running an SSG, so the hammer has a gap to the valve stem all the time (even uncocked)…. I could actually use a much lighter valve spring now, but the MRod spring is working fine and I can't be bothered changing it.... The required hammer strike with the smaller MRod poppet with the 1/8" stem is so low the gun is easy to cock, no need to change.... If you don't know about SSG's, stand by, this post will rock your world....  :o

Yup--my mind is blown by your SSG, Bob! I have a couple of hammer debounce devices (HDDs) that I got from Scot Laughlin several years ago. I've never installed them and maybe they'll stay in the box. Your work with the SSG looks like it might be a superior approach for me too once I get around to modifying my 397 for air retention.

BTW, before you started to delve into the ACP features, you were already roughly at Steroid power levels, based on my extrapolations from my own slightly less efficient 20 cal. Steroid (compared to 22 cal.) and also Tim's numbers from his old GTA posts...and unlike Crosman, Tim doesn't lie and exaggerate his Steroid performance levels!

One thing that readers should keep in mind is, according to maplogs.com, you live at 2438 feet. The standard pressure lapse rate is about 1 inch of mercury per 1000' elevation so you have to pump almost 9% more than you'd have to pump at sea level to realize the same pressure. (A steroid rated maximum of 14 pumps becomes 15+ pumps at your elevation.)

Sorry to hear your 392 had Crosman's had too-deeply stamped markings on it too. It's a common problem but it sounds like you corrected it in a manner similar to my fix.

My conclusion is that this bored out valve, with a volume of 6.7 cc, is too small to be able to get a useful 3-shot bell curve without pumping.... By increasing the volume by using a 2-piece valve, that could probably be accomplished, at the expense of a lot more pumps for the initial fill....
How about installing a "tank" under the stock. I got a couple of 16g and 20g CO2 cylinders to try to fit up for that purpose. I don't know of anyone who's installed a 20 gram one yet but I think it might fit, if the underside of the pump hand guard is routed out a bit too.
Title: Re: My Benjamin 392
Post by: rsterne on January 13, 2020, 10:34:32 AM
Cal, I don't know if you saw the two .25 cal Pumpers I made combining a Disco tube and 392 pumps parts or not.... First I built a carbine, using a 20" MRod barrel, stock 392 linkage, shortened Disco tube and lengthened valve (6.1 cc).... That is chronicled in this thread on the GTA....

https://www.gatewaytoairguns.org/GTA/index.php?topic=37401.0

On page 3 of that thread shows my installation of a 16 gr. CO2 cartridge (21 cc) to quadruple the reservoir size.... I still have another one here....

(https://i378.photobucket.com/albums/oo221/rsterne/Millenium%20Pumper/IMG_2753.jpg) (http://s378.photobucket.com/user/rsterne/media/Millenium%20Pumper/IMG_2753.jpg.html)

The success of that build (which used virtually all Crosman parts, and could have been made by Crosman for a tiny investment for the few new parts necessary) led me to build a longer version, which I called the Millennium Pumper.... It used a 25" TJ's barrel, full length Disco tube, Steroid 392 pump linkage, and a 2-piece valve even larger than what I had in the carbine.... It ended up at 28.5 cc....  :o

https://www.gatewaytoairguns.org/GTA/index.php?topic=39328.0

The MP could be tuned for one shot at 51 FPE (with a 40 FPE backup), two at 45+ FPE, or three at 40+ FPE with 25.4 gr. JSB Kings.... It had a Foster fitting for initial filling because it took 80 pumps from empty to reach the working pressure of 1800 psi.... If you repumped after a single 40 FPE shot, it was only 11 pumps, so not bad at all....  8)

(https://i378.photobucket.com/albums/oo221/rsterne/Millenium%20Pumper/IMG_2893_zps50d6fe30.jpg) (http://s378.photobucket.com/user/rsterne/media/Millenium%20Pumper/IMG_2893_zps50d6fe30.jpg.html)

I think an SSG could have saved a bit more air, and improved on that slightly.... maybe.... hopefully....  ;)

Bob
Title: Re: My Benjamin 392
Post by: rsterne on January 13, 2020, 01:19:45 PM
I tried to figure out what the efficiency was based on the pressure drop in the valve, for the first 2 shots at each pressure level.... It was a bit complicated, because I had to base the remaining pressure on the velocity of the third shot, but I think the chart below is pretty close....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Benjamin%20392%20Efficiency_zpsm0qr3qsg.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Benjamin%20392%20Efficiency_zpsm0qr3qsg.jpg.html)

I stopped the data at the SSG gap setting where the second shot was faster than the first.... The FPE I used was the total of those 2 shots.... and I calculated the volume of air used by taking the pressure drop (in bar) times the valve volume in CI (6.7 cc = 0.409 CI)…. This is similar to the way it is calculated for a PCP.... I confirmed by taking the total FPE when the gun was tuned for 2 equal shots and the number of pumps to top it back up, and the data is consistent with that....

The other way of calculating the efficiency of a pumper is to use the volume of air compressed by the pump (swept volume of 3.54 CI times the number of pumps)…. which always results in a lower value.... The difference is because of pumping losses, and the cooling of the air after pumping.... For the 4 pressures tested, at the SSG setting where there were 2 equal shots (3 for 6 pumps), I got the values below.... These values are always lower for high pump numbers in any MSP....

6 pumps..... 3 shots totaling 15.5 FPE requiring 4 pumps to replace = 14.2 CI.... 1.09 FPE/CI
10 pumps.... 2 shots totaling 21.8 FPE requiring 7 pumps to replace = 24.8 CI.... 0.88 FPE/CI
14 pumps.... 2 shots totaling 29.0 FPE requiring 10 pumps to replace = 35.4 CI.... 0.81 FPE/CI
20 pumps.... 2 shots totaling 37.2 FPE requiring 14 pumps to replace = 49.6 CI.... 0.75 FPE/CI

I'm pretty happy with these efficiency levels.... in particular the overall efficiency for when there were 2 equal shots, which is the way I would tune the gun most of the time.... That runs 1.29 to 1.43 FPE/CI, so the SSG is certainly doing its job.... 8)

Bob

Title: Re: My Benjamin 392
Post by: Insanity on January 13, 2020, 01:41:20 PM
Bob there is most definitely something wrong with you. Dont go see a Dr or anything and changing to normal on us lol.
Title: Re: My Benjamin 392
Post by: Alan on January 13, 2020, 05:23:23 PM
Incorrigible! At least that is what "I" think!

Bob, you're an example for the rest of us to follow. Unfortunately, darn few have the requisite skills you employ everyday. We're not just awed, we're jealous!
Title: Re: My Benjamin 392
Post by: rsterne on January 13, 2020, 07:10:46 PM
Today I decided to make an adjustable piston.... The valve has a cone on the front, and if I machined it off flat, like you would for a 13XX valve, and then made a matchng flat-topped piston, it would have exposed the check valve.... I'm pretty sure it would have still sealed OK, but the diameter of the exposed Delrin would be about 7/32".... I had one like that on a 2200 valve years ago, and it partially extruded through the hole.... After careful examination, I decided that I could machine the cone to a much shallower angle, which would eliminate some of the distance from the O-ring to the end of the valve, and shorten the small hole that feeds the check valve.... Both of those will eliminate some headspace.... Then I made the adjustable piston with the same angle, but concave instead of convex.... The angle that worked the best to leave the smallest lands near the O-rings, and not expose the check valve, was 20 degrees.... Here is what the end of the valve and piston look like....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Angled%20Piston%20and%20Valve%20Small_zpsmm05pxkq.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Angled%20Piston%20and%20Valve%20Small_zpsmm05pxkq.jpg.html)

If you look closely at the center of the valve, you can see that the check valve hole is so short you can see the end of the check valve.... When I machined the piston I started out with a slightly shallower angle, pressed the valve against it, and of course it rocked on the center of the cone on the valve.... I kept changing the angle, about 1/2 deg. deeper at a time, until the valve would no longer rock when pressed against the end of the piston.... This should work like an FTP, but with shallow matching angles.... The total distance between the O-ring on the piston and valve are the same as what the land on the valve was by itself.... It is actually less than the distance from the valve O-ring to the edge of the original cup seal was.... I don't see how I can reduce the headspace any more without the possibility of ruining the valve by machining into the check valve....

Here is the adjustable piston.... I even used an O-ring grooving tool with an hemispherical end to minimize any air gap under it.... Just behind the O-ring I machined a groove 3/8" wide and 0.10" deep.... This is wound with 3 layers of kitchen string (with the ends pulled underneath to secure them)…. This works like the felt on a 140 piston, and is soaked in a 50/50 mix of "Moly-Slip" transmission additive and 10W motor oil.... I use that mix for almost anything that needs lubrication, (except inside HPA reservoirs, of course) and have used it to soak the felt (or string pad) on many pumpers in the past, including my Millennium Pumper.... I talked to the 4th owner of that gun the other day, and he says it still functions perfectly....  8)

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Adjustable%20Piston%20Small_zps1o06hkbw.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Adjustable%20Piston%20Small_zps1o06hkbw.jpg.html)

I adjusted the length of the piston before I installed the O-ring and string pad.... With the valve installed I made it so that I could not quite install the pump pivot bolt.... At the moment the length is such that the pump handle stops about 1/2" from the tube.... Once things settle in I may lengthen it 1/4 turn to get a small amount of preload.... The adjusting threads are 1/4"-28NF, so 1/4 turn is only 0.009".... Now to assemble it and see if it takes fewer pumps to get to 1500 psi....  ???

OK, I put it back together, and I'm getting about 50 psi more at 20 pumps.... I have saved nearly 1 pump at 1500 psi.... There is very little difference at lower pressures, but it is definitely more efficient over 1000 psi.... I pumped 20 times, adjusted the SSG to 5 turns out, and I got 2 shots within a 2% ES, averaging 705 fps with the 18.1 gr. JSB Heavies.... It only takes 13 pumps to refill for another 2 shots.... That reaches my initial goal of 2 shots at 20 FPE....  8)

Bob

Title: Re: My Benjamin 392
Post by: calinb on January 14, 2020, 01:32:03 AM
Cal, I don't know if you saw the two .25 cal Pumpers I made combining a Disco tube and 392 pumps parts or not.... First I built a carbine, using a 20" MRod barrel, stock 392 linkage, shortened Disco tube and lengthened valve (6.1 cc).... That is chronicled in this thread on the GTA....

https://www.gatewaytoairguns.org/GTA/index.php?topic=37401.0
Hi, Bob. No, I missed your carbine project and your follow-on rifle. I spent the morning reading both threads and this evening I got caught up here. Impressive! I would consider duplicating the carbine, but I don't have a Disco tube and buying piecemeal parts from Crosman gets a bit expensive too. I do have a 392, an extra brass 392 valve and some parts from Mac1, including Tim's standard length Steroid pump arm kit. Maybe I'll leverage what I'm learning from you here and mod my 392 similarly. I have several small pressure gauges, including one with a 2000 psi scale. I've always wanted an FX Independence but I never spend that much money on my air rifles!

On page 3 of that thread shows my installation of a 16 gr. CO2 cartridge (21 cc) to quadruple the reservoir size.... I still have another one here....
If you decide to fit a cartridge up to the 392 and would like to try for a bit more volume, I'll send my spare 20 g. cartridge to you. Maybe that approach is too similar to what you've already done, but I have two of them. Just let me know and I'll mail it up to you. The 20 gram-er is a tight fit in length without interfering with the pump forearm, but a bit of wood can be removed from the underside of the pump forearm too, as I recall, in order to accommodate the rounded bottom of the cartridge. I was planning to place the 1/16 NPT fitting for it and the gauge very close to the exhaust valve (much closer than I show in my photo attachment above), but it would probably require either threading the 1/16 NPT through the existing 2-part valve threads or shortening the number of threads on the male valve half (which would also increase valve volume a little :)). I was hoping the latter would pencil out.
I'm pretty sure it would have still sealed OK, but the diameter of the exposed Delrin would be about 7/32"
Not that it matters but I'm pretty sure Tim said he uses Teflon check valves in the Steroid and he makes two of them from a single Crosman 392 valve. So your 392 valve should be Teflon too, unless I missed somewhere in this thread where you swapped it out for Delrin. (I saw your photo where you turned the end of the stock check valve for a spring seat). Tim uses Delrin for his exhaust valves. BTW, if you machine your valve seat to fit-up one of Tim's exhaust valves, you'll gain a small amount of valve volume and you'll never need to worry about extrusion of the stock valve stem rubber. I can send you one of Tim's Delrin exhaust valve stems along with the 20 g C02 cartridge, if you think you might be able to use a new exhaust valve too. :)

Title: Re: My Benjamin 392
Post by: rsterne on January 14, 2020, 09:02:40 AM
I don't think I want the volume of a CO2 cartridge conversion for this gun.... If 27-28cc volume was enough for 2 shots of 45 FPE in .25 cal, then roughly half that, which I can obtain with a 2-piece valve, should be sufficient for this .22 cal.... I mean I am there already, with just 6.7 cc of volume, which frankly amazes me.... I don't want to produce an 80 pump gun for initial fill, like the MP was (which is why I fitted a Foster)….

You probably missed what I did with the poppet on this gun.... I used an MRod poppet, and converted the valve body to accept its 1/8" stem.... It was a brilliant solution, and one I have used in other guns, such as the Disco.... Sorry for calling the check valve Delrin, you are likely correct, it is probably Teflon.... If that is the case, even more reason NOT to enlarge the diameter of the opening, as Teflon is softer and easier to extrude than Delrin....

Bob
Title: Re: My Benjamin 392
Post by: calinb on January 14, 2020, 11:07:33 AM
I don't think I want the volume of a CO2 cartridge conversion for this gun.... If 27-28cc volume was enough for 2 shots of 45 FPE in .25 cal, then roughly half that, which I can obtain with a 2-piece valve, should be sufficient for this .22 cal.... I mean I am there already, with just 6.7 cc of volume, which frankly amazes me.... I don't want to produce an 80 pump gun for initial fill, like the MP was (which is why I fitted a Foster)….

You probably missed what I did with the poppet on this gun.... I used an MRod poppet, and converted the valve body to accept its 1/8" stem.... It was a brilliant solution, and one I have used in other guns, such as the Disco....
Sorry for calling the check valve Delrin, you are likely correct, it is probably Teflon....
If that is the case, even more reason NOT to enlarge the diameter of the opening, as Teflon is softer and easier to extrude than Delrin....
Bob
No need to apologize, Bob. I noticed your mention of the poppet swap after I last posted. I know Tim has said Delrin for exhaust and Teflon for intake works best so I wanted to mention it. I understand the problem with the initial fill, if the gun tends to leak (or eventually starts to leak). On the other hand, assuming both are tuned optimally for the same desired FPE, a larger tank gives one the option of the same number of shots as a smaller tank with easier pumping (when the larger tank is operated at lower pressure). Always trade-offs, right?

I've always thought it funny that AirForce promotes the smaller tank of the Escape line of guns as a better survival gun configuration, because the smaller Escape tank is easier to fill with a stirrup pump than the larger gun tanks. Well yeah...the first time it's easier, but the larger Condor tank can produce the same number of shots at the same FPE with EASIER pumping than the Escape, if the Condor is run at lower pressure! On the issue of manual pumping, the Condor is is the better "survival" gun!
Title: Re: My Benjamin 392
Post by: rsterne on January 14, 2020, 01:19:13 PM
While a larger valve helps increase the FPE at the same pressure, for a retained air pumper.... the increase in volume works like a larger plenum in a PCP (which it is), rather than a larger reservoir (which it isn't, it's simply not big enough for that)…. I try and get a plenum of 1 cc per FPE when possible, but the penalty in pressure if the plenum is only 1/2 cc per FPE is typically about 8% (let's say 100 psi for a 20 FPE 292)…. When you go larger than 1 cc per FPE, the gains are not worth it, basically no matter how large you make the plenum you won't be able to reduce the pressure even 5%.... My goal is 2 or 3 shots at 20 FPE, so while the ideal plenum would be 20 cc, anything over 10 is good enough for me.... I will probably end up at about 2/3 cc per FPE, I hope (about twice what I have now)....

I'm already at 2 shots, so if I can double the plenum I hope to get 3 within an acceptable ES.... I'm not sure I trust hanging a rather large CO2 cartridge on the end of a 1/16" NPT fitting.... and I'm not crazy about trying to fit a 1/8" NPT fitting into a 392 valve.... Realistically, going from a 2-piece valve to what I have plus a 16 gr. CO2 might allow 50 psi less for the same FPE.... at the expense of twice as many pumps for the initial fill.... The number of pumps to replace each shot won't change much, because the efficiency won't change much, and you still have to pump the air back in that you used up for the shot....

Yeah, it's a matter of trade-offs for sure.... Plus I already did an external CO2 cartridge in the .25 carbine, I want to try something different....  ::)

Bob
Title: Re: My Benjamin 392
Post by: calinb on January 14, 2020, 08:10:38 PM
I don't think I want the volume of a CO2 cartridge conversion for this gun.... If 27-28cc volume was enough for 2 shots of 45 FPE in .25 cal, then roughly half that, which I can obtain with a 2-piece valve, should be sufficient for this .22 cal.... I mean I am there already, with just 6.7 cc of volume, which frankly amazes me.
Yes--amazing on 6.7 cc! The cartridge and Foster fitting impose two more potential leak points too, of course, and you've implemented a cartridge in the past so I understand your reluctance to unnecessarily complicate this gun when your goals are already met.

While a larger valve helps increase the FPE at the same pressure, for a retained air pumper.... the increase in volume works like a larger plenum in a PCP (which it is), rather than a larger reservoir (which it isn't, it's simply not big enough for that)…. I try and get a plenum of 1 cc per FPE when possible, but the penalty in pressure if the plenum is only 1/2 cc per FPE is typically about 8% (let's say 100 psi for a 20 FPE 292)…. When you go larger than 1 cc per FPE, the gains are not worth it, basically no matter how large you make the plenum you won't be able to reduce the pressure even 5%.... My goal is 2 or 3 shots at 20 FPE, so while the ideal plenum would be 20 cc, anything over 10 is good enough for me.... I will probably end up at about 2/3 cc per FPE, I hope (about twice what I have now)....

I'm already at 2 shots, so if I can double the plenum I hope to get 3 within an acceptable ES.... I'm not sure I trust hanging a rather large CO2 cartridge on the end of a 1/16" NPT fitting.... and I'm not crazy about trying to fit a 1/8" NPT fitting into a 392 valve.... Realistically, going from a 2-piece valve to what I have plus a 16 gr. CO2 might allow 50 psi less for the same FPE.... at the expense of twice as many pumps for the initial fill.... The number of pumps to replace each shot won't change much, because the efficiency won't change much, and you still have to pump the air back in that you used up for the shot....
The way I look at a plenum in the airgun world is it's just an HPA source with low impedance, whereas a tank does not necessarily feature low impedance, e.g., a tank behind a regulator. At what point does a large plenum become a tank?

I'll probably fit up a cartridge to my 392 someday. I'm not too worried about it, because it will be pretty well shrouded by the wooden stock and I'll need a 1/16 NPT adapter for the 1/8 NPT, pressure gauge anyway so I might as well drill a hole in the side of the fitting for the cartridge. Actually, a 1/8" NPT would fit in the muzzle side half of the valve but then a 12 g cartridge is probably the limit, due to interference with the pump arm (and it may still be the only option).

A high resolution gauge is useful for development work, but it's not actually necessary operationally, because recharge pressure can be adequately measured by the amount of pump arm rebound off of retained head space air pressure (even a very small amount that always exists). The desired final charge pressure can monitored as either a discrete set value using a spring loaded tell tale, or even a calibrated scale that can be pasted on the inside surface of the pump arm, as Steve Woodward has demonstrated. I won't be sure which way I'm going until I finish some other projects and find the time to work on it but your thread has got me thinking about my 39x guns again, Bob!
Title: Re: My Benjamin 392
Post by: rsterne on January 14, 2020, 10:37:56 PM
I readjusted the piston 1/4 turn tighter.... The pump arm rebounds about 1.5" from the tube, the bottom of the wood forearm is below the stock, so not quite as much rebound as with the rubber cup.... That has reduced the headspace a bit (just that 1/4 turn gave another 50 psi at 20 pumps)…. Here is a new velocity and pressure vs. pumps chart, with 1st, 2nd and 3rd shots charted.... I was running 2 turns of gap on the SSG, which I tested and found to produce no velocity drop with the 15.9 gr. pellets, even at 20 pumps....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Benjamin%20392%20Vel%20Press%20Adj%20Piston_zps5vcdgha2.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Benjamin%20392%20Vel%20Press%20Adj%20Piston_zps5vcdgha2.jpg.html)

I am now getting 1500 psi (maybe 1510, it is above the line on the gauge, not below it) on 18 pumps.... With the rubber cup that took 20 pumps, so I have definitely increased the pumping efficiency....  8)

I also tested it at 20 pumps with the 25.4 gr. Monsters, and I got 707 fps, which is 28.4 FPE.... I think that surpasses most of the Steroids I have seen numbers on, so I'm pretty pleased with that.... I also tried it a 20 pumps with 3 turns of gap on the SSG with the 15.9 gr. pellets, and I only lost 3 fps on the first shot, but the second shot was 691 fps instead of 640, and the 3rd shot was 326 fps instead of 210.... So, even if you tune for full power, backing off on the SSG will retain more air, in this case about 200 psi less used on the first shot, yet no loss in power.... That means fewer pumps tp refill....  8)

I made a much lighter hammer today too, and did some brief testing.... It weighs 40 grams (instead of 65), and I can no longer get full power with it at 20 pumps.... The reduction in weight means I need more spring to get the same power in the gun.... I tried it at 18 pumps (1500 psi), and instead of requiring 5.3 turns of gap like the stock hammer to get 2 equal shots, with the light hammer I only need 1.5 turns.... I got 4 shots with the 15.9 gr. pellets of 726, 731, 562 and 245 fps.... and the report is noticeably quieter.... The gun makes a nice crisp SNAP on the first shot and a louder BANG on the second shot, but NO burp of hammer bounce at all.... You don't get any with  a properly tuned SSG anyways, but the lighter hammer is definitely quieter and more efficient.... With the heavy hammer the third shot is about 550 fps, and there is not enough air for a 4th shot (just a pop, not enough to drive the pellet)…. This is proof positive that the lighter hammer / SSG combination is saving air....  8)

Bob
Title: Re: My Benjamin 392
Post by: Alan on January 15, 2020, 03:50:49 AM
zWith all of the new-found energy, the question remains, how well does it group?
Title: Re: My Benjamin 392
Post by: rsterne on January 15, 2020, 07:32:48 PM
With a little luck, I may find out in about May....  ::)

Bob
Title: Re: My Benjamin 392
Post by: steveoh on January 15, 2020, 09:52:39 PM
May? Why, it was a balmy 51 degrees here in northern California today.  Brrrr.
Title: Re: My Benjamin 392
Post by: rsterne on January 15, 2020, 10:16:11 PM
Bully for you.... it was -20*C here today.... that's -4*F....  ::)

Bob
Title: Re: My Benjamin 392
Post by: Alan on January 16, 2020, 04:42:42 AM
Don't pay any attention, Bob. He''s just bragging again.
Title: Re: My Benjamin 392
Post by: rsterne on January 18, 2020, 08:05:03 PM
I started working on the new valve front end today to convert to a 2-piece valve.... The body for the front end is made from scratch, turned from a piece of 6061-T6 aluminum.... The space between the O-rings is 1/2", to give room for the gauge port and the two low-profile 8-32 high tensile screws.... They are set down on spot faces on the valve, so that the head will be supported by the brass tube.... I made the check valve from a piece of 1/4" Teflon rod with a light spring.... The valve spring is the Conical spring I use inside an MRod valve.... In order to provide a spring seat for both, I made a thin stepped washer, and it is supported in a column high enough to provide about 3/8" preload on the valve spring.... I can shorten the column if that is too much.... The tublular column only has a 1/32" wall so that it doesn't take up much volume inside the valve....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Front%20and%20Springs%20Small_zpsfrlkg7o5.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Front%20and%20Springs%20Small_zpsfrlkg7o5.jpg.html)

The seat for the check valve is an 8-32 setscrew, threaded into the front of the valve so that the cup point just protrudes into the check valve hole.... It is drilled though with a 3/64" drill to minimize head space, glued in with Loctite 638, baked to cure it, and then machined off flush with the flat front on the valve....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Valve%20Front%20End%20Small_zpsinmqfigr.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Valve%20Front%20End%20Small_zpsinmqfigr.jpg.html)

I still have to drill the holes in the tube for the mounting screws and then I can check for leaks before I do the final machining on the back of the valve to shorten it just in front of the seat.... I also have to make a new flat faced piston for the pump....

Bob
Title: Re: My Benjamin 392
Post by: rsterne on January 19, 2020, 06:24:52 PM
Today I drilled the holes in the tube to mount the front part of the 2-piece valve.... It was mostly hand work again, because of the difficulty of holding the soldered together tube and barrel assembly in my small milling attachment on the lathe....  ::)

Once I got it mounted properly, I then made a new flat head for my adjustable piston to match the valve.... I didn't yet machine off the back of the valve in case there was a problem, and I'm glad I left it so that I could back up to the previous valve.... I struggled for over an hour trying to get the valve to seal in the tube.... I was sure it was the front O-ring on the rear valve half, and replaced it with a 90D, but it still leaked.... I finally gave up and went to reinstall the previous 6.7 cc valve, and while it was out I checked the poppet and it was leaking a bit.... enough that I couldn't get pressure into the gun....

I spun the poppet against the seat with a drill and it sealed right up.... I then tested the gun with the new FTP and valve front, but with the rear half of the valve still intact.... I estimate the volume at about 9 cc.... Anyways, it seems like the new FTP has less headspace, because it pumps better at high pressures.... Here is a chart of the velocity and pressure at up to 24 pumps with this interim setup....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Benjamin%20392%20Vel%20Press%209cc%20FTP_zpskcd0xoxn.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Benjamin%20392%20Vel%20Press%209cc%20FTP_zpskcd0xoxn.jpg.html)

I was using 1 turn of gap again on the SSG to make sure the velocity was peaked.... I'm quite pleased with the results.... The velocity with the 15.9 gr. pellets now peaked at 871 fps at 1580 psi, which is 26.8 FPE.... That is 2 FPE more at about 20 psi less pressure because of the increased valve volume....  8) …. With the 25.4 gr. Monsters I got 720 fps (29.2 FPE), and I will have more volume yet when I cut off the front part of the rear valve half just at the seat....  ;)

Bob
Title: Re: My Benjamin 392
Post by: calinb on January 19, 2020, 08:29:52 PM
<snip>
I struggled for over an hour trying to get the valve to seal in the tube.... I was sure it was the front O-ring on the rear valve half, and replaced it with a 90D, but it still leaked.... I finally gave up and went to reinstall the previous 6.7 cc valve, and while it was out I checked the poppet and it was leaking a bit.... enough that I couldn't get pressure into the gun....

I spun the poppet against the seat with a drill and it sealed right up.... I then tested the gun with the new FTP and valve front, but with the rear half of the valve still intact....
So was the leak from the poppet all along, Bob (instead of the o-ring)?
Title: Re: My Benjamin 392
Post by: rsterne on January 19, 2020, 09:13:55 PM
Yeah, and I never suspected it, because it has been fine all along.... With the 2-piece valve the two inside O-rings now have to hold pressure (rear on the front half and front on the rear half)…. so I figure it HAD to be one of those, or the new check valve....  ::)

WRONG !!!!

Bob
Title: Re: My Benjamin 392
Post by: rsterne on January 20, 2020, 08:00:13 PM
Today I machined off the rear part of the valve at the seat.... Here is the 2-piece valve, ready to install....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/2%20Piece%20Valve%20Small_zpspl453j14.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/2%20Piece%20Valve%20Small_zpspl453j14.jpg.html)

The volume between the pieces is 11.9 cc, but by the time I subtract the poppet, spring and spring seat (both parts), it drops to 10.8 cc.... The check valve and spring fill the space in the front section of the valve, so essentially that part of the valve is solid ahead of the rear O-ring.... Here is a chart showing the velocity and pressure vs. the number of pumps....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Benjamin%20392%20Vel%20Press%2011cc%20FTP_zpsnbtzawmt.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Benjamin%20392%20Vel%20Press%2011cc%20FTP_zpsnbtzawmt.jpg.html)

The pump is a bit more efficient than the version with the double cone that I used with the 6.7 cc valve.... I reached about 1630 psi at 32 pumps filling this 10.8 cc valve.... That resulted in a velocity of 904 fps with the 15.9 gr. pellets (28.9 FPE)…. I tested it at that number of pumps with a couple of heavier pellets.... The 18.1 gr. Heavies hit 870 fps (30.4 FPE) and the 25.4 gr. Monsters reached 770 fps (33.4 FPE)…. I was pretty happy with those results.... The SSG Gap was set at 1 turn to make sure the power was maxed out.... Using this adjustable FTP pump to fill a stock 4.2 cc valve would result in 1600 psi in just 12 pumps....  8)

I then decided to use just 24 pumps (about 1340 psi), with the 18.1 gr. pellets, and start increasing the SSG gap to conserve more air and see where I could get 2 equal shots, and what the power would be.... Here are those results....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Benjamin%20392%2011cc%20Valve%20SSG_zps0lfcq63o.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Benjamin%20392%2011cc%20Valve%20SSG_zps0lfcq63o.jpg.html)

There are some important things to see on that chart.... I can increase the SSG gap to 5 turns before the velocity of the 1st shot starts to decrease.... and adjusted like that there is enough air retained (about 950 psi) for a 2nd shot of over 700 fps and (about 450 psi) for a 3rd shot of over 500 fps.... At 6.5 turns of gap I get two equal shots of 740 fps (22 FPE), and at 7 turns all 3 shots get fairly close together, giving a 3-shot string of 692, 745 and 650 fps.... Unfortunately that is about a 13% ES, which isn't close enough.... so a 3-shot string doesn't seen to be within reach, at least not at over 20 FPE....

The two equal shots at 22 FPE was a bit more than I needed, so I reduced the initial fill to 22 pumps, which is about 1250 psi.... I found the perfect adjustment for the SSG at 7 turns of gap (1/2"), which allows me to shoot a single shot and pump 6 times to refill, or two equal shots and pump 13 times to get back to 1250 psi.... If I needed a 3rd shot for a Coup de Gras, there is enough air for a 600 fps shot remaining after the first two shots.... Here is a sample, with a mixture of single and double shots....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Benjamin%20392%202%20Shots%2018%20gr_zps2pcwrba5.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Benjamin%20392%202%20Shots%2018%20gr_zps2pcwrba5.jpg.html)

To say I am pleased with these results would be an understatement.... Not only did I achieve my goal of 2 equal shots, but I actually got 21.2 FPE at 1250 psi.... The gun is not too difficult to pump at that pressure, and it is a lot easier on all the components than 1500 psi would be.... As a retained air pumper, I can shoot continuously at 21 FPE with just 6 pumps to refill between shots....  8)

Bob
Title: Re: My Benjamin 392
Post by: Alan on January 21, 2020, 04:36:53 AM
I've run out of adjectives!
Title: Re: My Benjamin 392
Post by: Capt45 on January 21, 2020, 06:23:26 AM
Well, that's just STAR TREK results Bob.  Beam me up Scotty, I'm on board.  Now to do my homework and figure out how to manipulate my "financial" accts.
Title: Re: My Benjamin 392
Post by: calinb on January 21, 2020, 09:49:52 AM
<snip>... the 25.4 gr. Monsters reached 770 fps (33.4 FPE)…. I was pretty happy with those results....
I'll say! A 22 cal. Steroid only produces only 25 fpe when shooting even heavier 28.5 pellets. (Tim's numbers, but I've found his 20 cal. numbers to be truthful in my Silver Streak Steroid Chrony testing.)
<snip>... so I reduced the initial fill to 22 pumps, which is about 1250 psi.... <snip>
As a retained air pumper, I can shoot continuously at 21 FPE with just 6 pumps to refill between shots....  8)
Okay, to compare to a 22 cal. Steroid again (the next closest thing I've seen in maximum power), I estimate that your 21 FPE would require about 9 or 10 pumps after each Steroid shot/dump of air (10 pumps at Colemont's elevation) and also much heavier 25 gr. pellets, instead on the lighter 18 gr. pellets you used during this reduced (1250 psi) power testing. Once gain, this is an impressive result that illustrates greater versatility than I've previously seen in any Benji pumper project.

This is the most complex Benji pumper mod project I've ever seen, but the results are clearly worth it. At the risk of complicating it even further ;), two questions:

1. Do you think you could get better air transfer if you cut a seat for a small annular o-ring around the rear valve half port, as others have done? (The rear o-ring could then be optionally eliminated too.) Currently, the poppet's discharge must pressurize the volume between the two o-rings.

2. How much discharge air is lost past the poppet valve's metal stem portion? It's probably hardly any air at all, if the stem fits extremely precisely in its bore, but I'm curious about what you think, Bob.

-Cal
Title: Re: My Benjamin 392
Post by: rsterne on January 21, 2020, 04:50:43 PM
I don't know how I would cut an O-ring groove around the exhaust port, even if there was room to do so.... The surface of the valve, and the tube it fits into, are curved, so the O-ring groove would have to be much deeper on the sides than in the middle, relative to a flat plane tangent to the valve at the port.... I suppose a CNC could be programmed to do it, using a 1/16" or smaller end mill, but I don't happen to have one of those handy....  ::)

I am getting a blast of air out of the side valve holes in the rear half, but when I am finished development I will squeeze some silicone into the recess around the screws before I install them.... I hope that will eliminate that leak.... The volume between the 2 O-rings on the rear valve half is only about 0.2 cc, (figuring a 0.010" gap all the way around the cirfumference and a 1/2" width between the O-rings).... With a 10.8 cc valve, that is only going to cause an additional 2% pressure drop before the air gets to the pellet, so I don't think that is a big concern, once I get the leak stopped.... A fair amount of air is blowing out past those side screws....  >:( ….The MRod valve stem is a pretty good fit in the new insert I made for the back of the valve, so I don't think that is a major loss....

Today I fitted the lightweight hammer that I made the other day.... Now that I have reduced the operating pressure to 1250 psi, it has plenty of adjustment for the 22 pump initial charge.... At that pressure I was getting about 807 fps with the 15.9 gr. pellets (23 FPE)…. With 1 turn of gap on the SSG with the 18.1 gr. Heavies, I got 788 fps (25 FPE), which I'm pretty sure is maximum power at that pressure.... It doesn't really matter anyways, because I will be dialing it down until I get 2 equal shots.... Here is a photo of the two hammers....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Lightweight%20Hammer%20Small_zpset4sa1as.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Lightweight%20Hammer%20Small_zpset4sa1as.jpg.html)

The hole for the hammer spring is drilled 0.16" deeper than the stock hammer, so that I can set the SSG for zero gap with the adjusting bolt a turn clear of the back of the hammer group at zero gap, instead of 0.20" behind it.... That shortens how far the SSG protrudes.... The rear of the hammer is about 7/16" OD, and the front portion is only 5/16", which reduced the weight to 40 grams.... In addition, I machined the sear catchment collar on an angle to match the remachined sear, so that the load won't just be on the corner.... I made the hammer out of 1144 stressproof steel, but I didn't bother to harden it.... We'll see how long it lasts....  ;)

I started out at 22 pumps, with the SSG set for 1 turn of gap, and then increased the gap, recording the velocity of all 3 shots, and the pressure after the second shot.... Here are those results....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Benjamin%20392%20Light%2022%20Pumps_zpsfv4e7ky2.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Benjamin%20392%20Light%2022%20Pumps_zpsfv4e7ky2.jpg.html)

As was the case the last time I tried this lightweight hammer, the report is noticeably quieter, particularly on the second shot.... The first 2 shots were equal at 3.5 turns of gap (instead of 7 turns), and at that setting the remaining pressure had increased nearly 100 psi with the lighter hammer.... The velocity of the 3rd shot increased from 585 fps with the heavy hammer to 615 fps with the light hammer, proof positive it is saving air....  8)

It still takes 6 pumps to replace the air used by the first shot, but instead of 7 pumps on the 2nd shot, that has dropped to 6, so to refill after two shots only takes 12 pumps.... This also means that instead of filling to 22 pumps and getting 2 shots, I can just use 16 pumps for the initial fill, and shoot a 21 FPE shot and pump 6 times to refill, and do that for as long as I want to shoot.... ;)

I then repeated the process using 16 pumps as the initial fill (1000 psi), and dropping down to the 14.3 gr. Express pellets.... I started at 5 turns of gap on the SSG, and gradually increased that to 8 turns, recording the results as follows....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Benjamin%20392%20Light%2016%20Pumps_zpsdx9g4hpm.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Benjamin%20392%20Light%2016%20Pumps_zpsdx9g4hpm.jpg.html)

The results are similar, just at 16 FPE instead of 21 FPE.... The first 2 shots are equal at 1 flat less than 7 turns of gap, at 712 fps (16.2 FPE) average.... Just like yesterdays results, I can shoot a mixed bag of single or double shots.... It takes 9 pumps to refill after 2 shots, and 4 or 5 after a single shot.... As long as I fill to the high side of the 1000 psi mark on the gauge, all shots are within a 3% ES.... I also have the option of simply filling to 750 psi (12 pumps) and shooting a single shot with a 4 or 5 pumps recharge between shots.... I then repeated the process again, using only 12 pumps for the initial fill (750 psi), again with the 14.3 gr. pellets.... Here are those results....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Benjamin%20392%20Light%2012%20Pumps_zpsvjwpncx7.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Benjamin%20392%20Light%2012%20Pumps_zpsvjwpncx7.jpg.html)

Two equal shots occur at 2 flats past 9 turns of gap, at 620 fps (12.2 FPE)…. It only take 6 pumps to refill after 2 shots, or just 3 pumps after each shot.... I also found a setting that gave a decent 3-shot string of 585, 611 and 574 fps (11 FPE with a 6% ES) from that 12 shot fill.... It takes 7 pumps to refill to 750 psi to repeat that 3-shot string....  8)

I tried something interesting at this setting as well.... I pumped just 10 times, took a shot, and then refilled with 3 pumps.... I shot a total of 15 shots like that, averaging 608 fps (11.8 FPE) and the ES over all 15 shots was less than 2%.... This means as a retained air pumper I am getting nearly 12 FPE on just 3 pumps....  8)

I also checked the efficiency, based on the amount of air pumped into the valve at the three power levels where I was getting 2 equal shots.... At 12.2 FPE I was getting 1.11 FPE/CI, at 16.2 FPE I was getting 1.06 FPE/CI, and at 21.1 FPE I was getting 0.99 FPE/CI, based on the air pumped into the gun.... Calculated using the pressure drop in the valve over those two shots (like you would a PCP) it was between 1.3-1.5 FPE/CI....  :o

So, I can use the heavy hammer and about 30 pumps and hit over 30 FPE with a 25.4 gr. pellet.... Alternately, I can use the light hammer and tune for 2 equal shots of 21 FPE at 22 pumps with 18.1 gr. pellets, or 16.2 FPE at 16 pumps with 14.3 gr. pellets, or 12.2 FPE at 12 pumps with 14.3 gr. pellets.... In any of those three tunes, I can get a single shot at only 16 pumps, 12 pumps, or 9 pumps respectively.... Lastly, I can get a 3-shot tune at 11 FPE filling with 12 pumps, or shoot indefinitely at that setting from a 10 pump fill (just over 600 psi), with 3 pumps per shot giving 11.8 FPE....  8)

Bob
Title: Re: My Benjamin 392
Post by: calinb on January 21, 2020, 06:08:48 PM
I don't know how I would cut an O-ring groove around the exhaust port, even if there was room to do so.... The surface of the valve, and the tube it fits into, are curved, so the O-ring groove would have to be much deeper on the sides than in the middle, relative to a flat plane tangent to the valve at the port.... I suppose a CNC could be programmed to do it, using a 1/16" or smaller end mill, but I don't happen to have one of those handy....  ::)
More than one 39x modder has had good luck just cutting it square and flat. I have plans to do it properly, though I too lack CNC. (Someday I will spring for a DIY system for my already heavily mod'd mini-mill.) Lacking CNC, I plan to mill the o-ring surface with the valve chucked-up on my mini rotary table. The imports from India that Harbor Freight sells are very useful and also cheap. It would probably fit on your lathe milling attachment. (I have one of those too.) I even set up my spare valve on the table on on my mill and did a dry run for the o-ring using my DROs, but my family move to an old homestead in Idaho got in the way of completion and other project too.

If I ever find time to finish my homebuilt airplane, which is already painted even and waiting for its instrument panel "smoke test", I'm flyin' up to to the Mozey On Inn, assuming that I can manage to do it before you retire from the motel biz.

Today I fitted the lightweight hammer that I made the other day.... Now that I have reduced the operating pressure to 1250 psi, it has plenty of adjustment for the 22 pump initial charge....

I like! This power range is fun and easy to pump!
Title: Re: My Benjamin 392
Post by: rsterne on January 23, 2020, 03:52:58 PM
Today I decided to mount a Reflex Sight.... I had a set of Benjamin Intermounts, and they would have worked by placing them against each other, and just ahead of the receiver.... However, the sight line was pretty high using those, and that is right where I hold onto the gun when pumping it.... I also had the identical sight with a Picatinny mount (instead of the dovetails of the intermounts), and a short piece of Picatinny rail left over from another project, just a bit longer than the sight base.... I realized that if I machined a groove in the bottom of that, and screwed it onto the top of the receiver, that I could mount the sight significantly lower, and behind the loading port, where it wouldn't interfere with holding the gun while pumping, and was still far enough forward to miss the bolt handle....

I mounted the piece of rail in the milling attachment in my lathe, and milled it off flat on the bottom and then using a 3/8" end mill I milled a slot that was 0.090" deep.... When placed against the receiver, it sat on the bottom corners of the slot, with the top of the slot just clear of the top of the receiver.... I drilled three mounting holes in it, and milled pockets in the bars between the slots for the head of a 4-40 SHCS.... I then carefully laid out and tapped three matching holes in the top of the brass receiver.... It is 0.150" thick on top, and by shortening the SHCSs to 0.30" I was able to fasten it on securely, and have the screws clear the bolt so that it would still function properly.... It looks like this installed....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/Picatinny%20Rail%20Small_zps6ipkc9ad.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/Picatinny%20Rail%20Small_zps6ipkc9ad.jpg.html)

The front of the rail is even with the back of the loading port.... The large rear mounting hole in the rail is not used, it was just there on the piece I had left over.... It is actually long enough to mount a small scope, should I decide to do that.... Here is the finished gun with the Reflex Sight mounted and the hole for the gauge stained walnut to match the rest of the stock....  I also shortened the guide rod of the SSG so that it is flush with the preload adjusting nuts, since I will never have to adjust those again.... With the gap adjusting bolt moved forward because of the deeper spring hole in the hammer, you don't even notice the SSG when the gun is cocked, and my hand isn't in a position where it would get pinched on firing....

(https://i378.photobucket.com/albums/oo221/rsterne/Benjamin%20392/392%20With%20Reflex%20Sight%20Small_zps8wffmaaw.jpg) (http://s378.photobucket.com/user/rsterne/media/Benjamin%20392/392%20With%20Reflex%20Sight%20Small_zps8wffmaaw.jpg.html)

I grabbed a tin of 15.9 gr. JSB Hades pellets, as I wish to try those on the local Ground Squirrels in the spring.... I fiddled around with the SSG gap and ended right back at 3.5 turns, the same setting as I used for the 18.1 gr. Heavies, which means no tuning is required to switch back and forth between the two pellet types (although a resight may be required)…. I ended up with two equal shots of 765 fps (20.7 FPE) by using 22 pumps (1250 psi) or a single shot at the same velocity using 16 pumps (1000 psi)…. It takes just 6 pumps to refill for each shot fired.... I fired a mix of single and two shots, and over a dozen mixed shots I got an ES of under 3%.... The best consistency (lowest ES) was achieved by filling with 16 pumps, and then using 6 pumps to top up for the next shot.... Most of the shots just a few fps either side of the average, and the lowest and highest shots were only 10 fps apart.... When using 22 pumps and taking 2 shots, the first shot is noticeably quieter.... but the 2nd shot isn't terribly loud either.... about what you would expect for an unsuppressed 20 FPE PCP.... Incidently, while I had the gun apart today I inspected the sear and the hammer, and there was NO wear on either....  ;)

I think I'm pretty well done with this project, and I'm extremely pleased with the results.... There are several different tunes available, everything from this tune and lower just by adjusting the SSG gap to work with the pressure I pump to.... For maximum power, I need to drop the heavy hammer back in, and I can achieve over 30 FPE with the 25.4 gr. JSB Monster pellet at 30 pumps (just over 1500 psi)…. I find that to be a bit too much of a workout for this 71 yo, and the gun is so much easier to pump at 1250 psi, and the performance is great, I'm going to leave it that way.... The flexibility of two shots from that pressure, or a single shot from 1000 psi with just 16 pumps, and only 6 easy pumps to refill is just soooooooo great, I don't see a reason to use anything else.... but the flexibility is there if I want it....  8)

Bob
Title: Re: My Benjamin 392
Post by: Alan on January 24, 2020, 05:51:39 AM
Well, the question remains.... What more COULD you do? I think not much!

By the way, I like the rail installation.
Title: Re: My Benjamin 392
Post by: rsterne on January 24, 2020, 08:54:32 AM
Well, I could try and come up with a 2-shot magazine, and mount it on the front end of the scope rail.... That's why I mounted it flush with the back of the loading port.... Naw, I think I'll go lie down until that idea goes away....  ::)

Bob
Title: Re: My Benjamin 392
Post by: Alan on January 24, 2020, 09:49:12 AM
It is nice to know that you (still) have a sense of humor!
Title: Re: My Benjamin 392
Post by: rsterne on January 24, 2020, 12:47:54 PM
I'll add that to my (very long) list of projects to do when I'm bored out of my skull and have absolutely nothing else to do....  ::)

Bob
Title: Re: My Benjamin 392
Post by: rsterne on January 29, 2020, 09:26:56 AM
I actually had a look at a sliding 2-shot magazine for the 392.... One thing that concerned me was that it would stick out on one side (or the other) and I was concerned that since I place my hand on the top of the receiver above and forward of the loading port it might make the gun uncomfortable to hold when pumping.... However, before I even got as far as placing something there to see if that was a issue I measured the bolt diameter and found out that it was 0.28", even though this is only a .22 cal.... Since the bolt has to slide through the magazine during loading the pellet into the breech, that obviously was a no-go.... There is no practical way I can think of, of holding a .22 cal pellet inside a 0.28" hole and yet allow the bolt to slide through easily when cycling....

So, I can comfortably forget about that concept....  ::)

Bob
Title: Re: My Benjamin 392
Post by: Alan on January 29, 2020, 12:21:26 PM
One thing I wonder about. If you had a brand-new FX of some sort, what would you do with it?
Title: Re: My Benjamin 392
Post by: rsterne on January 29, 2020, 03:51:32 PM
I dunno…. Learn?....  ???

Bob