Cesiumsponge
Member
It would be interesting to see what velocities could be reached.
Likely nothing too fancy if you're shooting just a regular firearm in space using the same propellant. If you look into the hydrogen and helium type light gas guns we use for R&D, you can get velocities of 4-5 miles/second typical. I don't believe you can get much higher than that using hydrogen gas as a propellant, which is already as light a propellant as you can get.
The evacuation of air helps quite a bit I would think otherwise they would not do this in the lab. As an dynamic model, the bullet is travelling down the barrel and pushing air out of the barrel at a rate faster than the air can move, which would be the speed of sound at STP, as the propellant gases have a much higher speed of sound due to the intense heat energy (and possibly lighter molecular gas? I have no idea what the gaseous molecules are in a blackpowder rxn)
Its going to be signifigantly more than 14.7psi as the bullet travels down the barrel continually compressing the plug of air in front of it, so it wouldn't be a static 1atm force acting against the front bullet. Correct me if I am wrong on that though.
I still think an issue is metal brittleness at extremely low temperatures. As mentioned, it would fluxuate and heat up extremely fast with any radiation source, and without an atmosphere to vent heat via convection, you'd have to depend on the firearm as a blackbody radiator to dissipate heat.