Originally Posted by What's_nitro?

Antimatter:

PROs: Reacts with regular matter emmiting 100% photons at 100% efficiency, projectile should easily reach the Earth's escape velocity, excellent muzzle flash (once), no residue (no gun either), very little fuel needed per shot (a few billionths of a gram).

CONs: Chamber could possibly comprise of rings of high speed rotating magnets (expensive) to avoid fuel contact with chamber walls (bad), hard to transport (expensive), hard to obtain (expensive), may result in a negative effect on the Earth's natural rotation and balance (bad), very dangerous (a helmet and eye protection should be worn).

Yes, I know. I added some things...

It's a toss-up between antimatter and hairspray...