Mark French, a professor at Purdue University, created this cannon that'll accelerate a fragile ping-pong ball up to Mach 1.2--that's 900mph. This video goes into some nice physics-y depth about the logistics of getting a thin sphere of plastic up to that speed, and then the payoff at the end is just great. Turns out a 900mph ping-pong ball blasts clear through a paddle. [YouTube via Gizmodo]
I imagine if you create an explosive cartridge witch makes pressure and fires its pressure into a cartridge with a bullet in it with a vacuum, the vacuum cartridge shaped like the above rocket exhaust picture above, you might get a much more accelerated bullet with a stronger impact.
Of course, I am just day dreaming here....
That's an interesting idea Robot.
I'm not an expert in physics, but my suspicion is that you could indeed get a higher muzzle velocity as it exited your gun, but the bullet would be too fast for stable flight and would start to tumble and slow pretty quickly.
But perhaps with some experimentation you could come up with some sort of bullet-with-fins design that would work.
I would have liked to see what the ping pong ball looked like after.
Would it be possible to just use one tube to suck air from the vacuum part of the gun and push it into the other chamber with pressurized air? That would make it a lot more 'efficient' and usable :D