You would need to load the shell in a room that was pure O2 at high pressure and then make it air-tight. A better propellant would be HHO gas (Browns Gas) as it is fuel and oxidizer that has a flame travel that creates a 'crack' sound rather than a 'boom', that should mean a very fast muzzle velocity. 12V and some stainless steel is all it takes to create the gas.
BTW, do not use it in a paintball gun, . . .