Just an aside to point out the velocity of change from solid to gas changes with different types of powder. Most of us know that even powders of the same chemistry with different burn rates will produce the same amount of gas, but in a different time rate which means different pressures.
Honestly I though the original question was going to be about replicating firearm ballistics with a pressurized gas source.
If I remember correctly, the deflagration max velocity is 5,200 fps for the expanding gas (which is why they were trying to reach 5,000 fps with the Eargenshplitten Loudenboomer...). Anyways, the only firearm I know of that actually gets close is the smoothbore main gun of an M1A1 (or A2) Abrams main battle tank.
I was pretty tired when I wrote that original reply, so I probably didn't make a point so much as a ramble.
Machine guns are awesome until you have to carry one.