If you don't know what a rail gun is, it basically fires a solid projectile using magnets, at very high velocity. These projectiles contain no explosive; they are simply fired so fast that the impact completely destroys the target, in theory.
The initial tests were at around 10,000 megajoules, and fired these projectiles at over 5,000 miles per hour. There are videos and more details here:
The amazing thing, though, is the way the projectiles leave a firey trail in their wake, as far as I can tell essentially igniting the air they pass through with the sheer heat of friction. It makes the AIR BURN, people. *drool*