GURPS ULTRATECH ballistics?

[Anthony]

Originally Posted by DouglasCole

I used the ideal gas law to figure out how the pressure falls off as the volume expands (the volume is the chamber plus the barrel) and then just integrated Force = Pressure x bullet area over the barrel length.

Pretty sure that doesn't work once the expansion rate is significant relative to the speed of sound in the gas; ideal gas law is for equilibrium.