The AirGun Guild

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

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

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

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....



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.... 

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.... 

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.... 

Bob

With a little luck, I may find out in about May.... 

Bob

Bully for you.... it was -20*C here today.... that's -4*F.... 

Bob

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....



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....



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

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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)?

Today I machined off the rear part of the valve at the seat.... Here is the 2-piece valve, ready to install....



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....



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.... 

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....



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....



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.... 

Bob