Speed of sound
The speed of sound in a gas varies with temperature, hardly with pressure (there is a slight variation since gases are not Ideal). It also varies depending on the gas -- the speed of sound is a lot higher in hydrogen at room temperature than air (1270m/s vs. 330m/s). The interstellar medium is mostly hydrogen, I believe so even if it's a lot cooler than Earth's biosphere then the speed of sound will still be a lot higher higher than in air at STP.
The guns with the highest muzzle velocity use hot hydrogen behind the projectiles since the speed of sound is the limiting factor determining the final muzzle velocity (i.e. the speed of the pressure pulse behind the projectile).
Biting the hand that feeds IT
