A while back I wanted to try some 50-58 gr. cast bullets in my 2560, during some penetration testing I was doing, and I was shocked to find them down in power (from 50 FPE to 40 FPE) compared to the 25 gr. JSB Kings the gun was tuned for.... I thought at the time perhaps the bullets were too tight a fit in the barrel, and didn't have the time to follow up on it.... During the recent development of my QB79 Ninja, I recorded the velocity and FPE for a variety of different bullet weights, and then selected the most accurate (the 15.9 gr. JSB Exact Jumbo) to tune the gun with.... I tuned the gun for best effciency with that bullet (ie I tuned the gun to be operating on the "knee" of the velocity curve shown below), which required a significant reduction in the hammer spring preload, lowering the velocity just a few fps but resulting in a huge increase in shot count.... Then I ran a few more tests with lighter and heavier pellets and I found something interesting.... The lighter pellets didn't lose any velocity at all compared to the "full power" tests, while the heavier pellets lost more velocity, and the heavier they were, the more they lost from tuning the gun for the 15.9 gr. pellets.... Here is what I think is happening, using the graph for the way the gun was tuned....
With the 15.9 gr. pellets, the gun was operating on the knee of the curve, at 4.5 turns out on preload.... With a lighter pellet, the gun was still operating on the plateau on the left of the curve, so no velocity was lost.... With a heavier pellet, the gun was operating on the downslope on the right of the curve, so there was more velocity loss, and the heavier the pellet the more it moved to the right.... This meant that if you have a regulated PCP tuned to operate on the knee of the curve with a given weight of pellet.... if you change pellet weight you will have to readjust the preload to get back to the knee of the curve.... Lighter pellets will require backing off on the preload, and heavier pellets will require increasing the preload....
I decided to test this theory by going back to the 2560, which is currently tuned for 53 FPE with 25 gr. JSB Kings, and is operating just below the plateau of the curve, where the knee just starts.... I measured the velocity and calculated the energy with three weights of pellets with the following results....
25 gr. - 53 FPE
36 gr. - 57 FPE
53 gr. - 48 FPE
I then turned the RVA in 4 turns and ran the test again....
25 gr. - 55 FPE
36 gr. - 65 FPE
53 gr. - 71 FPE
Yep, there is what I was expecting to see.... The gun was tuned for a 25 gr. pellet, and with a 36 gr. pellet it only picked up 4 FPE out of a potential 12 FPE.... When tested with a 53 gr. bullet, it LOST 5 FPE, instead of GAINING 18 FPE.... I didn't bother to graph the 36 gr. and 53 gr. pellets to find out where the knee of the curves were.... but it was pretty obvious that the 36 gr. needed at least a couple of turns more preload and the 53 gr. needed way more than that to get close to maximum velocity and energy.... This adds one more "rule" to tuning a regulated gun....
For every regulator setpoint AND PELLET WEIGHT there is an optimum hammer spring preload....
1. For a given pressure, too much preload just wastes air....
2. For a given pressure, too little preload loses velocity....
3. Heavier pellets require more preload....
4. Lighter pellets require less preload....
5. Larger calibers require more preload....
6. Smaller calibers require less preload....
7. Higher pressure requires more preload....
8. Lower pressure requires less preload....
All of these rules also apply to unregulated guns, of course.... they just show up in a different way.... as changes in FPE and/or fill, refill, and peak pressures....
Bob
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