1. The number of molecules in the volume of the air in the charge.
2. Its initial temperature.
...there's a popular number that's totally absent and irrelevant: Pressure.
In other words, although initial pressure (of course) influences how efficiently energy can be extracted from the charge and translated to muzzle energy, it has no influence on the total energy available, which sets an absolute limit on the energy that can theoretically extracted by adiabatic expansion from each bar-cc of air. About 0.18 fpe.
No matter how high the pressure.
This message has been edited by pneuguy from IP address 220.127.116.11 on Jul 5, 2012 4:23 PM