Hello, Thank you all for the help here. Here we go, I have a known solid weight start of the propellent. I know the PSI of a known volume chamber(After combustion) and Temperature, can I find density? So example Bullseye powder has a load of 0.336954332 grams (5.2 grains) The volume of the barrel is 0.39591921 in3 The pressure at this point is approx. 29,000 psi Burn temperature = 2100k I am thinking because the known mass of the air in the chamber, plus the mass of the powder can be used for the total mass in the density of the ideal gas equation. [itex]\rho[/itex] = MP/RT Is this assumption correct? Will there be a substantial difference if I did not use the ideal gas law? I don't have the slightest clue what model would be appropriate to figure this as a "real" gas. Any suggestions? This might make more sense in what it relates to. This is related to firearms and reloading. I am trying to figure out the density of the gas in the barrel just before it leave the chamber. This is part of a larger problem having to do with recoil and compensation, but this simple part I just can not seem to make connect.