.25 cal Disco Pumper Carbine

[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 hate you Bob.


j/k

Can you make me one now?

[rsterne]

Today I installed the reservoir.... It is made from a threaded 16 gr. CO2 cartridge mounted on the side of a gauge mount I made for this purpose.... With the mount, it adds nearly 21 cc to the volume of the valve, so the total is now 27 cc....



I modified the stock so that when it is installed it hides the reservoir completely.... I'm very pleased with how it turned out.... You can see the Foster fitting for filling it from a SCUBA tank or stirrup pump, on the side of tjhe tube, just above the gauge.... I'm really glad I added that.... For the initial testing I never even used the pump....



I also installed a Fitco 2-7 x 32 AO scope.... For pumping you can either hold the scope, or the neck of the stock, and I find tucking the butt under my arm works well....



I couldn't stand the suspense, so I cranked the RVA to the max, setting the Disco hammer spring just shy of coil bind, filled the gun to 1500 psi from my tank, loaded a 25.4 gr. JSB King and fired the first shot through the Chrony.... How does 836 fps, for 39.4 FPE sound?.... Yeah, I was pretty pleased too !!! .... Here is a graph of the velocity vs. RVA setting for the first and second shots.... Obviously there is no way the gun can dump 27 cc.... the pressure available for the second shot was 1000 psi with the RVA maxed out, and gradually increased to about 1370 psi with it 10 turns out.... At 2 turns out, I get 820 fps (38 FPE) on 380 psi pressure drop....



You can see that the curves cross over at about 3.6 turns out on the preload, and at that setting you get two shots of 780 fps, which is 34.3 FPE.... and the pressure after the two shots is about 960 psi.... That works out to about 1.10 FPE/CI, which I am very pleased with.... By backing off the RVA more and reducing the fill pressure, you can reduce that power level.... That will mean fewer, easier pumps to refill.... Filling to 1250 psi, with the RVA about 6 turns out, gives two shots of 30 FPE and requires refilling from about 750 psi.... I shot a series of shot strings at each RVA setting, and graphed all shots that were within 10% of the maximum for each string.... Here are those results....



You can see that as the hammer spring preload is reduced, you get more shots, a flatter curve, and the peak velocity moves to a lower pressure.... In addition, you will notice that the shots at the high pressure end get dropped from the string due to partial valve lock.... These strings aren't ideal for some of the RVA settings because I always started at 1500 psi.... In fact, that is not the way you would use the gun at lower power levels, you would fill to a lower pressure.... As an example, look at the blue curve for 6 turns out on the RVA.... It uses about 250 psi per shot, and the peak of the curve is at ~1150 psi.... If you filled to about 1400, you could get three shots of about 720, 745, and 720 fps, ending at about 650 psi.... That is very close to 3 shots of 30 FPE with an ES of about 3%.... getting 90 FPE on about 85 CI of air, or 1.06 FPE/CI.... That might be the nearly perfect setting for hunting, and you would just top up after 1, 2, or 3 shots depending on what you needed.... The better your first shot, the less pumping you have to do....

I plan on doing a bit more playing around before deciding on a final tune.... but I think I can call this project an unqualified success at this point.... I will also be keeping track of the number of pumps to refill, today I was lazy, using the SCUBA tank.... Oh, I nearly forgot, I tried some other pellets with the RVA maxed out.... Here are the results....

H&N FTT 19.3 gr.... 911 fps.... 35.6 FPE
JSB King 25.4 gr.... 836 fps.... 39.4 FPE
H&N Baracuda 30.9 gr.... 779 fps.... 41.6 FPE
EunJin Dome 35.2 gr.... 737 fps.... 42.5 FPE
EunJin Pointed 41.7 gr.... 678 fps.... 42.6 FPE

So there you have it, a .25 cal pumper that shoots over 42 FPE on 1500 psi.... built with mostly Disco parts, an MRod barrel, and a 392 pump linkage.... with a few custom parts thrown in.... and Eric, the answer, as you know.... is no....

Bob

[rsterne]

It was time today to fix a couple of things that cropped up during building and testing.... I had noticed that there was a problem with the hammer dragging during testing.... If I squeezed off a shot instead of "yanking" it and holding the trigger back, I lost about 40 fps and the velocity was very inconsistent.... For that reason, all the previous testing was done with the "yank and hold" technique.... I pulled out the hammer and found exactly what I had expected.... a dent.... circled in the photo below....



Remember I shortened the valve stem 0.060".... Well that extra hammer travel was allowing the hammer to impact the back side of the sear, if it was still sticking up like it would be if you just squeezed off a shot.... I've seen this before, that is why I knew what was going on.... I had another Disco hammer handy, that had been lightened 20% by removing the front half of the rear portion of the full diameter part, making the "waist" longer.... Therefore, the section where the impact could occur was no longer there.... Since I wasn't going after maximum power, and I had lots of RVA adjustment to play with, I simply dropped in the lighter hammer and that fixed the problem.... You could squeeze off a shot and get exactly the same velocity as if you "yanked" on the trigger....

The other small issue I addressed was the "clacking" of the pump arm lever against the tube on closing.... I took a small piece of 1/8" thick UHMW Polyethylene about 3/4" square and forced it into the end of the pump arm flush with the end and down all the way inside it.... It stuck up about 1/16" above the arm and forestock so that the plastic is what hits the tube.... I drilled through it for the rear pin to retain it, reassembled the forestock, and voila, much quieter on closing.... BTW, somebody asked about holding onto the scope while pumping.... I find it quite comfortable if I put my hand right over the front scope ring.... Holding the gun there I'm pushing straight down right at the ring, and in all the testing I did today I didn't notice any looseness or POI shift.... I don't think it's a problem....

Now it was time to see if the new hammer made any difference.... It wasn't a big difference in weight, but the valve should open slightly faster, have a bit less dwell, and close quicker.... I expected to see a slight loss in velocity , but in fact I gained a little bit at maximum preload, with the 25.4 gr. JSB Kings hitting 845 fps (40.3 FPE).... I guess the faster opening helped a bit, or possibly the stock hammer was just catching the sear before I could yank it out of the way before.... In any case, I'm now getting over 40 FPE with the Kings for one shot.... In addition, each shot is using slightly less air, so the second shots are a bit faster.... I get two shots of equal velocity with the RVA only 3 turns out, and consequently they are more powerful, at 796 fps (35.7 FPE) instead of 780 fps at 3.6 turns out previously....

Now it was time to get to the serious setting up of the gun.... I'll spare you all the tribulations, but I eventually settled on a 1400 psi fill and 6 turns out on the RVA.... Here are the results....



The 4th shot is included just for interest, although it could be used for a "coup de gras" if necessary.... The exciting part is that I get 3 shots within a 3% ES, averaging 31 FPE.... I was only testing indoors, but those three shots kept going into one ragged hole.... With a 3% ES, I wouldn't expect any significant POI shift out to about 70-75 yards between those three shots.... I also pumped the gun up after 1, 2, 3, and even 4 shots, and recorded the number of pumps, as follows:



It takes 12 pumps to refill after the first shot, another 15 for the second shot, and 18 each for the 3rd and 4th shots.... The pressure drop per shot is 190 psi for the first shot, 230 for the second, 320 for the 3rd, and 410 psi for the 4th shot.... That gives efficiencies of 1.39 FPE/CI for the first shot, 1.23 for the 2nd shot, 0.83 for the 3rd shot, and a dismal 0.49 FPE/CI for the 4th shot, which only leaves 250 psi in the gun.... Like I said, the 4th shot is for emergencies only, at close range, to finish off a wounded animal.... The average efficiency over the first 2 shots is 1.31 FPE/CI, and over the first 3 shots is 1.10 FPE/CI.... I'm pretty darned pleased with that.... It sounds like there is a bit of hammer bounce on the 3rd shot, and it is definitely present on the 4th shot.... but with the good efficiency on the first 2 shots it really can't be an issue....

I think I'll stop here for a while, and do some work on a PCP that has been calling my name.... In conclusion, with 25.4 gr JSB Kings, I have 3 tunes as follows:

Max. @ 1500 psi.... 1 shot at 845 fps.... 40.3 FPE
3 turns out @ 1500 psi.... 2 shots at 796 fps.... 35.7 FPE
6 turns out at 1400 psi.... 3 shots at 739 fps.... 30.8 FPE

One other thing.... When filling from a SCUBA tank, I would strongly suggest using a regulator set to just below where you want to fill (I'm using 1350 psi).... I damaged one gauge by overfilling the small valve by just 200 psi.... It wasn't a safety issue, just a PITA and cost me a gauge.... The regulator prevents that from happening again....

Call it a day, I'm done like dinner....

Bob

[rsterne]

I decided that I liked the way this gun responded to a lighter hammer, so I had another one kicking around that weighed only 40 gr. (30% lighter than a Disco hammer).... I installed it, and the gun responded in exactly the way it should.... I lost about 8 fps at the top end (with greater efficiency) with the Kings, and I needed one more turn of preload on the RVA to get back to my "3-shots at 31 FPE" tune.... and they are a bit more consistent, with a whisker less air use.... I got one additional benefit.... With the RVA at maximum, filled to 1500 psi, and shooting 42 gr. EunJin Points, I get two shots at 655 fps (40 FPE) that are only 2 fps apart.... Should I wish to tackle something larger and that requires extra penetration, that combination would be just the ticket....

Bob