...is why a fill of CO2 gives you about three dozen good shots and a fill of HPA at teh same pressure only gives you a few shots.
And it has to do with how c02 works. CO2 is sold as a LIQUID in our uses; it's sold by weight (12gr., 88gr. or in ounces in paintball tanks). Air is measured by PRESSURE.
(OK...in theory, if you pressurize any gas enough, it turns to liquid...actually, becasue gas heats as you pressurize it, would have to pressurize, suck out the heat, pressurize, suck out the heat, etc. Pressure to turn any gas into a liquid for anything other than co2 is giagantic and the end temperature would be so low, you'd have a very hard time finding a way to dispense it).
Co2 is a liquid. At least up until about 100degrees, part of the co2 in a cartriges is liquid. When you let some out (fire a shot) the liquid converts to pressurized gas, and that keeps the pressure equal in the storage container. Eventually, you'll shoot enough that all the liquid converts to gas, and that gas will be at something like 950-1000 psi at normal temperatures. SOOOO...you get a lot of shots while the C02 is liquid.
If you compare a tank of ALL GAS (no liquid) co2 at 1000psi (what happens when you finally run out of liqid co2) and air at 1000psi, co2 still gives more shots. It's heavier than air, so the valve can only transfer a small amount in the time it's is open. Air is "thinner", the same valve will let more of it escape (to push a pellet) in teh same time frame. So while you'd get fewer shots per volume of gas in the tube using HPA, the shots would be faster.
With air, could pump up the big air tank to 950psi and shoot without a regulator. Would get a lot fewwer shots on a tank ful as there is no liquid air to donvert to gas as you shoot. Could fill the same size tank with 950psi of co2 GAS (no liquid). Would stillg et few shots, but would get more than with HPA (becasue less co2 gas flows with each firing).
