The AirGun Guild

A friendly place to share one's interest in all things airguns.

I cleaned the barrel, removed the open sights and stuck a cheap scope on it.... filled it to 2000 psi and it seems to hold air OK.... so I ran the six different weights of JSB Exact pellets through it.... I did each Chrony string from 2000 psi, and stopped when the first shot dropped below 90% of the highest velocity, and here are the results.... For the number of shots within a 10% ES, drop off the last shot in each string.... so that is only 15-19 shots.... This is fewer shots than the original Disco, from the same pressure fill and at similar FPE.... despite having a slightly larger reservoir.... one of the drawbacks of a shorter barrel (21" vs. 24")....



Note that in every case the first shot was the fastest.... and only the first half of each string fell within my usual 4% ES.... Only with the 33.6 gr. Beasts was there a plateau before the decline.... This means that other than for that pellet (which IMO is way too heavy for a stock Chief), only the first 7-10 shots are within a 4% ES.... This is an indication that, at least in my gun, the hammer strike is a bit too much for the 2000 psi rated fill pressure.... In fact when the gun is filled to 2000 psi on the gauge, the gauge on my Great White says 2100 psi, so it's not a matter of the gauge on the gun reading low, either.... While I was testing, I also shot strings with my BBT 27.4 gr. HP and BBT 29.6 gr. FN bullets.... They are "as cast", and measure 0.217" at the head and 0.219" at the driving band.... I have no idea what the barrel on the Chief measures, but their velocity and energy fit right in with their weights compared to the 25.4 gr. JSB Monsters and 33.6 gr. JSB Beasts.... so I can only assume they are about the right size.... The strings would have fallen right in between those two lines on the chart above.... and they both had a plateau of about 7 or 8 shots before the velocity started to drop....

I shot each string into a target 20 ft. away, at the other end of my shop, to get some idea of how the gun grouped.... as expected, all targets were one ragged hole (they better be at that range with 16-20 shots).... The worst group was the 13.4 gr. RS, the 14.4 gr. Express were much better, the 15.9 gr. Exact the best, closely followed by the 18.1 gr. Heavies.... The 25.4 gr. Monsters (the original design, I don't have any of the "redesigned" ones yet) were noticeably larger, and the 33.6 gr. Beasts were larger again, but still tighter than the 13.4 gr. RS.... The BBT HPs grouped the same as the 33.6 gr. Beasts, and the BBT FN were slightly worse, but still marginally better than the 13.4 gr. RS pellets.... The tightest group (15.9 gr. Exacts) was about 1/8" CTC for 18 shots, and the worst (13.4 gr. RS) was about 1/2" for 16 shots.... The 14.4 gr. Express and the 18.1 gr. heavies both did about 3/16" C-T-C for 17 shots, and clearly the middle weight pellets (14.4 - 18.1 gr.) shot the best from this barrel at this limited range.... From past experience, I would expect them also to shoot the best outside at longer ranges as well....

Here is a summary of the energy and efficiency with each pellet....

13.4 gr. JSB RS - average 854 fps / 21.7 FPE @ 0.86 FPE/CI
14.4 gr. JSB Express - average 845 fps / 22.8 FPE @ 0.89 FPE'CI
15.9 gr. JSB Exact - average 817 fps / 23.6 FPE @ 0.91 FPE/CI
18.1 gr. JSB Heavy - average 791 fps / 25.1 FPE @ 0.98 FPE/CI
25.4 gr. JSB Monster - average 712 fps / 28.6 FPE @ 1.11 FPE/CI
27.4 gr. BBT HP - average 689 fps / 28.9 FPE @ 1.12 FPE/CI
29.6 gr. BBT FN - average 671 fps / 29.6 FPE @ 1.13 FPE/CI
33.6 gr. JSB Beast - average 642 fps / 30.7 FPE @ 1.16 FPE/CI

The Chief has a 21" barrel, and I estimated the reservoir at 9 CI (~148 cc) to calculate the above efficiency numbers.... The ending pressure was over 1300 psi for the lightest pellets (shortest string), and over 1100 psi for the heaviest pellets (longest string).... and the gun used between 41-43 psi per shot.... All pressure readings were from the Beeman gauge, which has 200 psi increments, so estimating the pressure within less than 50 psi is in reality "guessing".... However, as you can see, the heavier the pellet the greater the total energy in the string, and the higher the efficiency, which is pretty typical for a PCP, particularly one that is "oversprung" and wasting a lot of air with the lighter pellets.... In reality, Beeman have opted for higher velocity numbers over a proper bell-curve, much like Crosman did when they "retuned" the Disco by adding a heavier hammer spring.... This is a wasteful way to tune a PCP, I wish Beeman had done better.... It is begging to be detuned a bit to trade off a bit of power for more shots.... but of course there are other alternatives.... Unless Beeman are hiding one inside, there is no external preload adjuster to easily accomplish a retune....

Now that I have about 150 shots through the gun, I can comment on the trigger, and action.... The bolt operates reasonably smoothly, and I prefer the "cock on open" to the original QB cocking, I don't like not being able to feel the pellet seat in the barrel when I close the bolt.... The trigger is a typical QB trigger, and although Beeman say it is a 2-stage, the trigger pull is heavy enough I really can't tell.... There seems to be a slight increase in trigger pull just before the sear releases, but the initial travel is long and hard, and there is a bit of creep in the "second stage" (if it even exists, I'm still not convinced).... Some work on the trigger will be the first thing I do.... and pulling the receiver off to see what the porting is like will happen pretty quickly as well....

Bob

I was thinking about the JSB Exact RS 13.4 gr. pellets.... they are not shooting much faster than the 14.4 gr. Express, and they are not grouping well (the poorest of the pellets).... They have a VERY thin skirt, and I suspect the skirt is blowing out of shape drastically.... If it is expanding over much of it's length, it would have more bore friction, and if the skirt is so thin it is expanding (unevenly) as it leaves contact with the muzzle crown, that could easily make it inaccurate.... It would be interesting to recover some of those, and the 14.4 gr. Express that DO shoot well and comparing them, if you could do so without distorting/damaging them.... Lloyd's crumpled up grocery bags stuffed into a 5 gal. pail should work for that....

I really don't care enough to bother, as I plan to improve the gun to where they would be much too light anyways.... but it could explain the lower than expected velocity and poor accuracy.... I have had Springers where the RS shot poorly, and actually shot SLOWER than the heavier Express did.... I suspected the same thing, as Springers have a high pressure peak, combined with a high temperarture.... perfect conditions for blowing out a thin skirt.... Food for thought, even though unproven?....

Bob

I removed the trigger, spring and sear, here is what they look like stock.... I rubbed the sliding surfaces lightly with 400 grit to highlight the imperfections for the camera....



The trigger is cast, and actually pretty smooth, although there is a bit of fine pitting on the surface that slides along the sear.... The sear, on the other hand, is a mess.... All three rubbing surfaces have heavy lines cutting across the contact areas.... The surface in the photo above slides across the top of the trigger after the sear releases, so not important for trigger feel, but it is for smooth hammer release.... The tiny spring plunger in the photo above will be used to create the second stage....



In the second photo, you can see the stock (heavy) spring, and a typical spring I might replace it with.... longer and lighter.... This photo of the sear shows that it is a real mess.... The upper surface is what locks the hammer in place.... those lines won't affect trigger feel, but they will affect smooth and consistent hammer release, and could even cause wear on the hammer.... You can actually see a burr on the top edge, right where it grips the hammer.... The lower surface is the one that is critical for smooth trigger pull.... Just look at all the lines running across it.... and imagine what that will feel like as you squeeze the trigger....

Time for some TLC with 220, 400, 600 and 800 grit wet-or-dry sandpaper.... It is critical not to change the SHAPE of the contact surfaces, just remove the roughness and polish them.... Stay tuned....

Bob

Nope, not for that price!....

Anyways, a half hour of work makes a WORLD of diference.... Here are the trigger to sear surfaces.... The short surfaces are the "active" ones that affect the feel and smoothness of the trigger pull.... the longer, curved ones slide over each other after the trigger breaks....



and here are the surfaces that contact the hammer.... The short one holds it back and the long one contacts during cocking....



I expect a huge difference, time to put it together, and install the lighter spring and 2nd stage plunger.... back soon....

Bob

Well, trigger mods always seem to need tweaking.... The lighter spring I used was too light, so I went back to the original one, at reduced preload.... This produced an "acceptable" trigger, but I really need a spring in between the two.... Using a longer, weaker spring with increased preload will give a more constant trigger pull over the 1st stage travel.... which is what I wanted.... but the light spring I had was just too light....

I noticed one other thing I'm not crazy about, and I have seen this before with QB triggers.... When you lighten the trigger spring, the sear has a tendency to not reset if you pull the trigger through the first stage and then release it without firing the gun.... The reason is that the load from the hammer is too great to overcome the friction between the sear and the trigger.... The term for this is "balk firing", and it is VERY common in Springers, where you usually have to re-cock the gun to reset the sear.... I can't tell you if the stock Chief trigger was doing this or not, I didn't check for it.... but the surfaces were so rough I would bet on it, even with the stiff spring.... It is something to keep in mind, if you start to take a shot and then change your mind, you can leave the sear "hanging" on the verge of firing.... The way to clear it is to recock the gun, by pulling the bolt back all the way to take the hammer load off the sear.... It is a good practice to get into, and especially necessary if your sear doesn't reset when you partially pull the trigger and then back off on the shot....

While I had the gun apart, I pulled the barrel and receiver and checked out the ports.... They are pretty small, and all the same size.... The barrel port, receiver port and valve exhaust port are all 3.5mm (0.138"), and the flat O-ring type seal between the valve and receiver is also the same ID.... In addition, the valve in my gun is slid back a bit, so the exhaust port is partially covered by the seal, restricting it even further.... That 25 fps that Ribbon had over my gun could be right there....  .... The valve has slid back because the holes in the tube for the valve screws are a bit larger than the screw heads.... With 2000 psi of pressure pushing back on the valve with 1178 lbs. of force, the valve slides back until the screw heads are tight against the back of the holes, and the exhaust port, that should have lined up with the seal and receiver port, is back from where it should be.... A seal with an oversized hole would probably help, or if, as I am, you plan to enlarge the ports, you can carve more out of the front of the exhaust port than the back, to re-center the port relative to the hole in the tube and the ports in the receiver and barrel....

Anyways, with the gun back together, I tethered it to a regulator set to 1800 psi and proceeded to go through the entire range of preload adjustment, shooting five JSB 18.1 gr. Heavies at each turn of preload and recording the average.... The results are in the chart below....



This is a classic curve, with a "plateau" where additional preload adds NO velocity and only wastes air.... a "knee" where the velocity starts to drop, and then a "downslope" where every time you reduce the preload the velocity drops.... Stock preload on my gun was 4 turns out, right at the beginning of the plateau.... Therefore adding more preload does nothing but make the gun louder and waste more air.... On the other hand, the knee of the curve is at 5-6 turns out, and that is a good place to experiment.... Less preload than that looses more velocity.... Now this was at 1800 psi, at the 2000 psi fill pressure, the entire curve would move up (faster) and to the left (more preload).... That 4 turns out stock setting is probably right at the top of the knee at 2000 psi.... Anyways, now that I knew where the knee was, I could try reducing the preload to see what happened to the shot strings.... Here are the results....



Only shots within 4% of the maximum velocity are shown on this chart.... The red string is from yesterday, but I omitted the shots below that 96% velocity point, leaving only 8 shots within a 4% ES, the velocity dropping with every shot.... pretty dismal the way it came from the factory.... This is what happens when you tune a gun on the plateau.... I reduced the preload 2 more turns, and shot the string in black at 6 turns out.... The starting velocity was only 742 fps, it wasn't until shot #5 that the velocity climbed within 4% of the 794 fps peak (which is why the first 4 shots have been omitted from the black line).... I got 22 shots, ending at 1400 psi, so the usable string was 18 shots within a 4% ES, starting at 1900 and ending at 1400.... I increased the preload a turn, to 5 turns out, and shot another string, recorded above in blue.... This is about as good as it gets for my 2000 psi fill.... The first shot was within my desired 4% ES, the string peaked at 809 fps, and I got 16 shots with the string ending at 1500 psi.... Here is the summary of these 3 strings.... all of which were shot with JSB 18.1 gr. Heavies, from a 2000 psi fill....

4 turns out - 8 shots, average 814 fps (26.6 FPE) @ 0.98 FPE/CI
5 turns out - 16 shots, average 794 fps (25.4 FPE) @ 1.31 FPE/CI
6 turns out - 18 shots, average 779 fps (24.4 FPE) @ 1.41 FPE/CI (1900 psi usable fill)

So, there is the key to tuning your stock Beeman Chief.... Try backing out the preload adjuster a turn or so, to get a proper bell-curve and double your usable shot count, with a big increase in efficiency....

Bob

I modded the gauge port and the inlet side of the valve this afternoon....



I milled three overlapping holes, offset, through the gauge block, and then removed the excess material with a Dremel.... the hole is about 7/32" x 1/2" and D-shaped with rounded corners.... There is still about 1/16" of material between the hole and the O-ring seat for the gauge.... The valve spring is larger than 5/16" ID, so I drilled the front end of the valve to that size.... The end of the valve, and both ends of the gauge block, are countersunk just to ease the flow through them.... I gained a 2-3 cc's of volume, so I will use 141 cc (8.6CI) for future calculations....

I carefully examined the valve attachment as well.... There is a steel backing block between the valve and the hammer, into which thread the flathead screw holding down the front of the receiver from the top, and the screw holding on the stock from the bottom.... That block in a QB is tight up against the back of the valve, so that those screws do some of the work in holding the valve.... In the Chief, there is a gap of about 1/16" between the back of the valve and the front of that block.... If you install the block and drop the valve in place, the back of the three tapped holes for the valve screws are flush with the back of the holes in the tube.... This means that the three valve screws are "on their own", holding all the force pushing back on the valve (1180 lbs. @ 2000 psi).... Those three 4 mm x 0.7 mm pan head screws look like they could be purchased at any hardware store that carries Metric, they certainly don't look of particularly high quality.... and the threads are in shear.... If the screws are at the minimum of the tolerance diameter at the thread root, that is only 0.118".... Assuming they are all taking equal load, I ran the numbers for three typical screw grades.... and the best SHCSs you can buy....

Grade 2 screws (74ksi UTS).... Safety margin, 60% shear - 1.2:1.... I REALLY hope Beeman isn't using cheap screws !!!
Grade 5 screws (120ksi UTS).... Safety margin, 60% shear - 2.0:1
Grade 8 screws (150ksi UTS).... Safety margin, 60% shear - 2.5:1
Metric Grade 12.9 screws (170ksi).... Safety margin, 60% shear - 2.8:1

This means that the screws should really be grade Metric 12.9 screws, which is basically the best SHCSs you can get.... The screws used are 5 mm long, so the proper McMaster Carr screw would be the following.... https://www.mcmaster.com/#91290a138/=1avzg91

I wish I had access to those screws, I would at least KNOW what they are, although they don't quite have the 3:1 safety margin I would like (and 3.5:1 is preferred).... I'm betting they are better quality than the pan head Phillips screws that Beeman use.... I could be wrong, of course, maybe Beeman had some special screws made, anything is possible.... One other advantage to the McMaster Carr screws is that the head is 7 mm diameter, which will be a far better fit in the 7 mm holes in the tube.... and way better than the 6.4mm head on the existing ones.... Incidently, in case you thought about shimming the backing block up to sit against the valve and share the load, I have a word of caution about that idea.... It could be done, if done carefully, so that the valve screws were still sharing the load.... HOWEVER, it would mean that you should not even remove the stock without degassing the gun.... If you make that block part of the load bearing system, then don't forget you did that.... I prefer having the proper screws to locate the valve, and then I can remove the stock and receiver without any concern about the valve anchoring system.... just the way it is supposed to work now....

Guess what.... There isn't a hope of me finding 4 mm x 0.7 mm short SHCSs locally, and McMaster Carr won't sell to Canada.... Rather than wait weeks to get someone to source them and forward them to me.... I am going to drill out the valve and fit three 10-32 low-profile SHCS, just like the whizzy mod you do for a Disco for higher pressure use.... Now this doesn't mean you can run more pressure in the Chief, we still don't know what the tube is made of.... In addition, the tube ID on the Chief is 22 mm (0.866") instead of the 0.745" on the Disco.... That means the Chief has 35% more force on the valve at the same pressure.... 2000 psi in the Chief is like 2700 psi in a Disco.... In fact, at 2100 psi, which is where the Chief (assuming a 1006 steel tube) has a 3.5:1 safety margin the three 10-32 low-profile SHCSs, with their 140ksi UTS, will only have a 3.7:1 safety margin.... That suits me just about perfectly.... I will have to drill the holes in the tube out to 5/16" for the larger screw heads, but that will decrease the stress in the bearing area.... While I am at it, I am going to drill the hole in the top of the tube to 5/16" as well, in preparation for a larger OD Teflon transfer port....

This project is finally starting to see some direction....

Bob

The next step was to fit the valve to the tube.... This involved careful setup to redrill the three valve screw holes and the hole for the port all at 90 deg. to each other and in the same plane.... All four holes were drilled to 5/16", to fit the heads of the 10-32 low-profile screws, and the new 5/16" OD transfer port.... Then the receiver had to be drilled out for the transfer port to pass through, and the barrel machined to accept the top of the transfer port.... Here is what the parts look like after this was done....



There were two additional modifications made to the receiver.... Note on the side, just below the notch for the loading port (above in the photo as the receiver is upside down) a 6-32 setscrew near the back of the notch.... There is a matching one on the other side.... These setscrews tighten against the sides of the back of the loading tray section of the barrel, greatly increasing the stability of the barrel in the receiver.... Also note that the M5 flat-head screw behind the transfer port is already in place.... It is trapped between the barrel and receiver, once the barrel is in place....

With the new Teflon transfer port, the barrel/receiver assembly must be lowered over the transfer port, which fits like a peg in the two holes shown in the above photo.... It will be about 0.010" longer than the space available, so that it is in compression when you tighten that flat-head screw.... The way this is assembled means you cannot install the receiver and then slide the barrel into place from the front afterwards.... I will admit that this was an unforeseen problem, but fortunately there was an easy solution....



The photo above is a top view of the barrel/receiver assembly.... Note there is a hole drilled in the barrel loading tray to allow access to the Phillips drive in the flat-head screw that holds down the receiver.... This enables you to drop the flat-head screw into the receiver, slide the barrel into place and fix it with the (now) three setscrews (the original M5 on top, and the two 6-32s on the sides at the back of the loading tray).... and then drop the assembly over the transfer port, tightening the flat-head screw (through the new hole in the barrel loading tray) as you lower it into place.... and finally using that screw to put the transfer port into compression to seal it.... I admit (sheepishly) that I was very lucky that this solution was possible.... otherwise I would have worked myself into a rather embarrassing corner.... Sometimes it is better to be lucky than smart....  .... Here is a photo of the barrel, modified with the new port, and showing the access hole for the flat-head screw....



There is a 5/16" diameter flat machined to accept the top of the Teflon transfer port.... It is machined down flush with the bottom of the O-ring grooves on either side of it, and breaks through into the sides of the O-ring slots.... If the O-rings were still needed, this would be a problem, but of course they are not needed now that the Teflon transfer port will seal directly against the barrel in compression.... I may still install them, they won't do any harm, of course.... but they really aren't necessary any more.... Here is another view of the new porting system....



Looking through the receiver and into the barrel port (on the left), you can see the oblong port machined into the barrel, and the way it blends into the 0.21" diameter circular shape at the TP parting face.... The port was machined with a 5/32" end mill, with the back of the port vertical and flush with the back of the 0.21" TP, and the front angling forward at a 30 deg. angle.... This makes the barrel port, at the borelilne, 0.17"W x 0.25"L, which gives it the same area as the 0.21" diameter round TP.... On the right, you are looking through the main tube, into the valve, and you can actually see right into the valve chamber, showing the vastly increased area for flow.... The only remaining job was to modify the bolt to thin the probe, the results of that are shown below....



Before starting changes on the bolt, I loaded a JSB 18.1 gr. Heavy pellet with the stock bolt.... The larger diameter but fatter probe pushed it just flush with the front of my angled barrel port.... However, because of the taper inside the pellet, and the smaller probe I wanted, I knew that the new probe would have to be longer.... It turned out that increasing the overall length of the bolt by just 1/16" was all that was needed.... However, the new, longer barrel port was so much longer that you could actually see the front O-ring on the bolt (there were two, tiny O-ring installed stock).... Before installing the new longer 3/32" probe (in a hole drilled in the end of the bolt), I machined off the bolt shorter, to remove the front portion of the front O-ring gland, leaving just the small diameter bottom portion of the gland intact.... The front of the bolt is now flush with the back of the transfer port, so that it will not obstruct the airflow in any way.... This leaves just the single rear O-ring in place to seal the breech.... I see no reason for two, that is a redundancy that I can live without (I hope).... I guess I will find out when I fire the gun for the first time.... If it does leak, I can always machine it to take a slightly thicker Metric O-ring, with more compression on it when in battery, to insure a seal.... The new, thinner probe seats an 18.1 gr. JSB Heavy just comfortably ahead of the new angled barrel port.... The area around it for airflow is still the smallest part of the porting system, but is the equivalent of a 0.200" diameter port.... compared to the 0.138" diameter of the original porting system.... This means I have increased the area of the ports by 110%.... so it is 2.1 times the area I started with....

Bob