I was explaining something in another thread, which made me look at this graph a different way, and I wanted to share it with you.... Here are the curves of velocity vs. hammer spring preload for my Grizzly at two different pressures, with three different pellets....



You can see that in every case, once you reach a certain preload, the velocity plateaus and no more velocity can be obtained.... As you reduce the preload, the velocity starts down slowly (what I call the knee of the curve), and then falls in a more of less linear fashion as you reduce the preload further (the downslope).... Think for a minute what is happening where two curves for the same pellet cross, for example the two red lines representing the 45 gr. JSB pellet.... The dotted red line (the velocity at 2900 psi) crosses the solid red line (the velocity at 1800 psi) at about 930 fps, when the preload is about 2.7 turns out from flush on my particular RVA.... This means that at that preload, if you started shooting at 2900 psi and shot a string down to 1800 psi, the string would start at 930 fps, climb to a higher peak, and then drop back to 930 fps at 1800 psi.... As you go to a heavier pellet, you need a bit more preload to get the velocity the same at both 2900 and 1800 psi, particularly when you go from pellets (45-50 gr.) to a 70 gr. bullet....

While this really won't help you tune an unregulated PCP, understanding what is going on sure can't hurt....

Bob