I had to spend a couple of days pruning our fruit trees for the winter, but the outside winterizing projects are now done, so I was back in the shop this afternoon.... I machined the barrel port on the .25 cal, 25" long barrel to match the new 3/8" OD x 1/4" ID transfer port, using an oblong barrel port of 3/16" x 5/16" at the boreline, so the porting is full bore area straight through.... I was expecting great things when I tethered the gun to my 1900 psi regulator, and I wasn't disappointed.... The first shots with JSB Kings were nearly 1100 fps.... I was able to use the same preload and gap setting on the SSG as I used in the .357 cal at the same pressure, and since that velocity was too high, I switched over to the 34.2 gr JSB Heavy Mk 2s.... The velocity was still nearly 1000 fps, with the gun doing 75 FPE at 1900 psi on PELLETS !.... The best I had ever seen before at that pressure with the old valve was 67 FPE, so the larger ports certainly did their job.... The ideal pressure with the Heavies is 1700 psi, which produces a velocity of 960 fps at an efficiency of 1.04 FPE/CI.... This was a lower efficiency than I was hoping for, but better than I have seen at such a low pressure before.... Here is the table with the normalized results for the 34.2 gr. JSBs....
I then switched over to a 50.6 gr. BBT, and ran two strings, one from 3000 psi down to 1900 (using a smaller SSG gap), and the other from 1900 down to just below 1500 psi.... At 3000 psi, I got 1018 fps (116 FPE), which is a bit more than I ever got with my own valve at the same pressure.... The big difference is that it developed that power on a LOT less air than my previous best setup.... Tethered at 1900 psi, I got 885 fps (88 FPE) at an efficiency of 1.30 FPE/CI.... By comparision, the unregulated version would do an 11 shot string at the same average FPE, but the efficiency was only 0.80 FPE/CI.... Backed off so that the peak velocity matched the Cothran valve, I could get a 14 shot string within a 4% ES, at 0.95 FPE/CI.... Mind you, I did not have an SSG fitted to the unregulated version.... Here is the table of normalized results with the 50.6 gr. BBTs....
I am quite pleased with the results with the Cothran valve in a regulated .25 cal PCP.... It can shoot 34 gr. JSB Heavies as fast as you would want to on just 1600-1700 psi, at pretty decent efficiency.... You could expect to get about 18 shots with a 13 CI (215 cc) tank, or 30 shots with a 22 CI (360 cc) tank.... If you used a 500 cc bottle (30.5 CI) it should be possible to get over 40 shots at about 950 fps.... I think that any of those combinations would be a dynamite Varmint rifle.... The plenum on my Disco Double is the entire upper tube (135 cc), and you really only need half that with the 34 gr. pellets.... A 2260 tube would be the perfect volume, and 1700 psi should do the job.... The added benefit is that the gun would be very easy to cock....
If you wanted a bullet shooter for the 51 gr. BBTs, you would need about 2000 psi to drive them at 900 fps (91 FPE), or 2400 psi to hit 950 fps (102 FPE).... Using a 3000 psi tank wouldn't give you a lot of shots at the higher pressure, but not too bad with a 2000 psi setpoint.... Of course if you had a 4500 psi tank, then you could get a ton of shots.... With a 500 cc bottle, you should be approaching 50 shots at 100 FPE.... The Cothran valve flows enough air to handle even heavier bullets, and a .257 cal shooting the Lyman 257420, which weighs 73 gr. should get into the mid 900s without too much problem.... If we assume that is attainable at 3000 psi, with an efficiency of 1.0 FPE/CI (which I think is nearly a given).... then a 500 cc 4500 psi bottle, regulated at 3000 psi, should get about 20 shots at 145 FPE.... While this is only theory until proven, I think it is a reasonable assumption, based on the testing I have done.... In his testing of the MRod Cothran Powerhouse Valve, which has the same port sizes, Lloyd got 950 fps with a 73 gr. bullet in .257 cal at 2800 psi (in a 30" barrel), with excellent efficiency.... There is no reason the Disco version should not do the same.... A 28" barrel at 3000 psi should have the same power as a 30" barrel at 2800....
Bob