Why? A quick review of high school physics is needed! First, let's talk about the familiar CO2 pistols that have been used very successfully for the last 15 years. And, by the way, these pistols are STILL just as good as ever! As I have been fond of saying, they didn't turn into "dog-doo" overnight! These guns were successful due to the fact that the vapor pressure over a liquid is a constant at any specific temperature. Therefore, as long as you had any liquid CO2 left in the gun's cylinder, you had the same pressure for the next shot as for the first.
The precompressed air guns work on an entirely different system and must obey the gas laws. In this case they are V1P1 = V2P2 and P = nRT/V. (V = volume, P = pressure, n = amount of air, R =the "gas constant" that makes everything come out right, and T = temperature). From this we see that, because the volume of the gun's cylinder stays constant, the pressure in the cylinder must change with each shot. The gun, however, is fitted with a regulator that is designed to "step down" the pressure in the cylinder to a more-or-less (depending on the efficiency of the regulator) constant pressure ("working pressure") that is presented to the pellet. Thus, until the pressure in the cylinder drops to the working pressure, you have the same velocity for each shot.
Now let's talk about the supply tank. Each time you fill a cylinder the amount of air ("n") in the supply tank is reduced, so therefore the pressure remaining in the tank is less. The pressure in the cylinder is almost the same as the pressure in the tank (a little less because V2 in this case is the volume of the supply tank + the cylinder + the connecting lines). Because you have less air after each fill, you will get fewer shots/less fill the next time and so on until you decide you need to get your tank pressure back up to 3000 PSI. How soon this is or "how many fills (shots) can I get" depends on the size of your tank. In my opinion, the 80 cubic foot standard US SCUBA tank is about as small as you would want. And how many shots per cylinder is entirely dependent on the pressure in the tank at the time. Some of you are wondering about the "T" in the equation above. Yes, temperature does have an effect. But, compared to the steepness of the CO2 vapor pressure vs. temperature curve, the curve for compressed air is pretty flat and can safely be ignored for the practical business of shooting. So, now you know!
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