I can't help you on the math side of things but from my days playing painful I can tell you to things: 1) (based on what i was told years ago) when the Co2 is filled, it is pressurized becoming a liquid instead of gas. This may or may be part of your trouble with calculations. 2) the rate of expansion from liquid to gas varies incredibly based on the temp of surroundings. So when it's hotter gas expands much faster. So when we chrono in the morning to make sure velocity was safe, by noon we would have to chrono again because.every one with co2 would be shooting hot at unsafe velocity.
Again sorry I can't help you with the math part of your problem. Just trying to lend you a hand from my past experience with co2 use.
As far as if they actually sell you 20 oz I don't think so. I think they fill it short so to allow for expansion. I can't tell you for certain and I am not airsmith.
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