.25 cal Disco Pumper Carbine

[SeanMP]

I like it!How about a 2 stage piston to keep the pump force under control

[rsterne]

After a few months of thinking and buying bits, and a few weeks of making parts, I finally got to test my .25 Disco Pumper Carbine built the way I originally envisioned it.... I was hoping to get 30 FPE with a 24" barrel, and I'm pleased to announce that I have achieved more than that with the 20" MRod barrel.... With the Disco hammer spring maxed out and the gun pumped to 1500 psi, it drives 25.4 gr. JSB Kings at 750 fps (31.7 FPE) and just barely dumps all the air.... With a 24" barrel, another 30 fps should be achievable, putting the gun over 34 FPE....

The modded valve is now 6.1 cc, and the 3/16" ports all the way through, along with a bit more hammer travel, allow the gun to dump a lot more air.... It takes a couple more pumps to reach 1500 psi because of the bigger valve volume.... Here is a graph showing what happens with the Disco hammer spring maxed out to nearly coil bind....



Note that the gun achieves 700 fps on 13 pumps, over 600 on 9, and over 500 on just 6 pumps.... I'm very pleased with the pumping efficiency, and now that I have the stock on the gun it's a lot nicer to pump.... The effort increases after 10 pumps, but it's still not ridiculously hard at 16, which is just shy of 1500 psi.... My .22 Uber-Pumper which used a 1400 tube and linkage was about the same effort at 20 pumps, from what I recall....

The next set of tests involved pumping the gun to 1500 psi and then dialling back the preload on the hammer spring a turn at a time with the RVA, all the way down to 10 turns out from coil bind.... Here is a graph showing the results....



The gun started retaining a puff of air at 2 turns out, but at 5 turns out it was still shooting 732 fps (30.2 FPE) with about 420 psi remaining in the valve.... so that works out to 27.7 CI of air used, for an efficiency of 1.09 FPE/CI (15 barcc/FPE).... It takes 13 pumps to get back to 1500 psi, and the pump displaces 54 cc, so based on the amount of air input (702cc) the efficiency is 23 barcc/FPE.... The difference is due to pumping and cooling losses.... If I crank the RVA out about two more turns, the velocity drops to ~690 fps (27 FPE) but now it only takes 10 pumps per shot.... At that setting, the second shot is over 500 fps, and if I turn the preload out to 9.5 turns, the shots are equal velocity at about 610 fps (21 FPE).... The pressure after the first shot is about 960 psi, and the second shot just dumps the air, or leaves a puff....

To summarize, this pumper can do nearly 32 FPE in a dump shot from 1500 psi.... Dropping the power down to 30 FPE saves 4 pumps, and at 27 FPE, you only need 10 pumps per shot.... It can achieve two shots of 21 FPE with the current size valve.... I'm extremely pleased with these results, they exceeded my original hopes for this design.... Now that I have tasted the benefits of retained air, however, there is another step to take.... I will be adding a 16 gr. CO2 cartridge to the valve, hidden in the stock, to increase the valve volume to a total of 27cc.... While this would require 70-75 pumps to reach 1500 psi from zero, I have already installed a Foster fitting so that the initial fill can be done from a SCUBA tank or stirrup pump.... The extra volume will drastically reduce the pressure drop during each shot, which should give either a boost in power, or more shots, depending on how it's tuned.... At that point, I don't know if it's a retained air pumper, or a PCP with an onboard pump.... but it looks like whatever it is, it should be fun finding out....

Bob

[AirGunEric]

I like the idea of retained air so as to reduce the number of pumps required for a second (or third shot).

Any idea when you will have it fully assembled for us voyeurs to see the final configuration with the gauge and caplet installed?

[rsterne]

I made the barrel band and have cut the stock and mounted the rear part.... I have to slot the foregrip for the linkage and install it.... Then comes boring and porting the valve.... and more testing.... At that point, you will see photos (with gauge).... I'll give you a hint, the overall length is only 35.5".... The extra reservoir comes after all that.... so it will be a while yet.... You won't see it anyway, it will be hidden inside the stock....

Bob

[AirGunEric]

I hate you Bob.


j/k

Can you make me one now?