The words you are looking for is not PSI, but ENERGY.
psivelocity ratios are not a correlation that can be figured out unless you have the volume of air determined.
Try this, figure out the volume and the pressure of a dump chamber type of gun, and then you can figure out the amount of energy used.
I already did this once, and have the file somewhere. It is about 45 pages long.
Surfice to say, a 'Mag has a .55 ci in dump chamber with a 400 psi operating chamber. It then uses about 210 ci*lbs of energy per shot (have this figured better on the other paper.) A Shocker has a 1.8ci dump chamber with a 180 psi working pressure. The shocker uses about 324 ci*lbs of energy. I have designed a pbgun and have some details on the PaintballNews>the 'Shiva' project. I have a dump chamber less than 1.4 ci, and a working pressure of 160 psi. I use about 225 ci*lbs of energy each shot.
To figure out how many shot per tank, take the volume, say 68ci, and the pressure, 3000psi, multiply.
68ci * 3000psi = 204000 ci*lbs
minus the lowest operating pressure of the gun, in my case 140psi, times the volume (68ci)
68ci * 140psi = 9520 ci*lbs
204000  9520 = 194480
divided by the amount of energy used each shot, and you get shots per tank.
194480 / 225 = 864
reverse this for a gun, say a spyder, which normally gets say, 800 shots on a 68/4500 tank, and runs down to 300 psi or so, and you can find the amount of energy used per shot.
Happy mathematics,
Josh
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