Author Topic: My Benjamin 392 (Read 4872 times)

Alan · « **Reply #60 on:** January 21, 2020, 04:36:53 AM »

I've run out of adjectives!

Capt45 · « **Reply #61 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.

calinb · « **Reply #62 on:** January 21, 2020, 09:49:52 AM »

Quote from: rsterne on January 20, 2020, 08:00:13 PM

<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.)

Quote from: rsterne on January 20, 2020, 08:00:13 PM

<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....

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

rsterne · « **Reply #63 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....

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....

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....

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....

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....

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....

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....

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....

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....

Bob

calinb · « **Reply #64 on:** January 21, 2020, 06:08:48 PM »

Quote from: 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....

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.

Quote from: rsterne on January 21, 2020, 04:50:43 PM

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!

rsterne · « **Reply #65 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....

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....

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....

Bob

Alan · « **Reply #66 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.

rsterne · « **Reply #67 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

Alan · « **Reply #68 on:** January 24, 2020, 09:49:12 AM »

It is nice to know that you (still) have a sense of humor!

rsterne · « **Reply #69 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

rsterne · « **Reply #70 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