Raimey - you aren't going to crack the speed of sound in a gun barrel. The powder gas is hot enough that you won't go that fast. Also, no amount of pressure will cause the gas to exceed Mach one (speed of sound for existing conditions and not a fixed velocity) in a straight wall bore. A convergent nozzle section can achieve Mach one, but you must add a divergent section to exceed Mach one. Excess pressure will only increase the density of the gas, but not the velocity, as you approach the speed of sound. Post back if any of that begs questions - I ought to be able to explain it.