I did this work a loonnngggg time ago, but it was never published here.... so I thought I'd post it in case some of you are interested.... The basis of the experiments was a Crosman "1750" which is a 2250 with an 18" long .177 cal barrel.... I made it up for indoor target shooting, and since this testing was done it has been detuned to under 500 fps for the Canadian rules.... First the gun....
It was equipped with a "Dual Power Hammer" or DPH made by Ken (CrustyB) in Canada.... Basically, it is a rear cocking hammer with two sear positions that gives two power levels.... and in addition an RVA with 10 turns of travel.... Here are the results, using JSB Express 7.8 gr. pellets, taking one shot every 30 seconds....
Full power (High Power 10) gives normal results.... ie about what you would expect with a "stock" 1750 or slightly more.... Note the declining velocity, and approx. 35-40 shots per cartridge.... Total FPE per cartridge was ~260 FPE (not including the dive at the end).... By backing the adjuster down to just under half way (to High Power 4), it is interesting to note that the starting velocity is nearly the same, and you lose less per shot.... You get an extra 20 shots per cartridge with an INCREASE in average velocity.... getting 55-60 shots per.... and a total FPE per cartridge of ~400 FPE (50+% greater).... Pretty impressive for an INCREASE in average performance....
Dialing it back to the lowest setting on the "high power" sear catch, the velocity dropped about 50 fps, but there was virtually NO cooling effect occurring, and a true plateau formed.... The shot count went up to 80-90 shots, extracting ~500 FPE from a cartridge.... Cranking the adjuster back in to setting 8 (out of 10) but using the "low power" sear catch, the velocity dropped to ~500 fps, but the shot count went up to 120-125 shots per cartridge (over 3 times original).... The total energy being extracted from a cartridge was now ~540 FPE.... well over DOUBLE that of a stock gun.... The shot string was basically flat....
One other interesting thing happened when I dialed back the hammer strike.... The gun became, to a large extent, self-regulating.... ie the velocity could be made independant of the temperature.... In fact, if you went really light on the hammer hit, you could reverse the normal trend, and make the gun shoot harder in high temperatures.... With such a light hammer strike, however, you couldn't pierce a new CO2 powerlet on low power.... Here is another interesting graph, showing how the gun performed at three temperatures, 75*F, 62*F, and 46*F....
These were done at setting "9" on the preload, and at both high and low power.... At high power (which could be considered "stock" performance), the velocity drops about 50 fps with each drop in temperature.... At low power, however, you can see that the velocity is largely unaffected by the temperature.... In fact, the lowest velocity was recorded at the highest temperature.... although the shot count went up.... This shows that with careful balancing of the springs in a CO2 gun, you can achieve a velocity independant of the temperature, or nearly so.... It requires a very light hammer strike to do so, and relatively low power.... but it CAN be done.... Once you find the balance point to make the gun self-regulating at CO2 pressures, then you can fine tune the velocity by varying the transfer port size.... This gun has since been set up with a smaller transfer port and a fixed, shortened, 2240 spring and a very light hammer to deliver about 130 shots at just under 500 fps over a very wide temperature range.... I have achieved over 600 FPE on a single cartridge.... and this gun is actually cheaper to shoot than a .22 cal springer even with the cost of the CO2 factored in....
Bob
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