You can fire a bullet without a gun, but it won't go very fast, or be lethal.
The main thing the gun provides that accelerates the bullet to lethal speeds is a chamber and barrel that can withstand the gunpowder's explosion, and then block all of the hot exploding gas from escaping, so that the only way for it to go out is to push the bullet down the barrel, and that's what gives the bullet its speed.
Basically, without a chamber and barrel, ammo is pretty harmless.