[MadMax]

Dude, you're dead wrong in asserting ideal gas behavior.

For instance when we shoot we let out some gas which is replenished by liquid evaporation. The ideal gas model would have the pressure drop steadily with each shot. The fixed volume example does not hold in our mags because we let gas out into the environment which lets "n" go down. In this situation we let "n" reduce as we allow some fractional mole of gas out of the container. If internal volume of the mag is constant (it is because it has non moving walls) and we allow temperature to remain constant (allow the mag to warm a bit between shots) then it would follow that pressure would have to drop unless you want to allow a change in the Boltzmann constant.

pv cannot equal nRt if only "n" changes. Therefore situation non ideal.

The ideal gas law neglects molecular size and molecular interactions which only holds as long as the average distance between molecules is large compared to molecular size. This breaks down considerably when you reach condensation conditions and molecules eqilibrate across a phase transition. In this condition molecular interactions are not negligible and even become dominant in dictating behavior.

Of course gas type will affect pressures. It will have a stronger effect under non ideal conditions than under ideal conditions because of molecular interaction. For instance a liter of gaseous air or propane or HFC134a at 1atm will all have similar behaviors when compressed to half a liter. Take a liter of any gas far from condensation conditions at STP conditions and compress them to half their volume and you'll get two atmospheres pressure. It's only when you get closer to condensation conditions for particular gases will they star to exhibit significantly different behavoir (other than other chemical characteristics like combustion). It is because we have non ideal conditions in our GBBs that we can switch propellants to achieve different operating pressures.