This is a video demonstration of the Navy's latest and greatest railgun, which looks suspiciously like the torn off member of a Decepticon if you ask me. "You're the expert." You're damn right I am, I spend time in the bathtub looking these things up on my phone.
The revolutionary railgun relies on a massive electrical pulse, rather than gunpowder or other chemical propellants, to launch projectiles at distances over 100 nautical miles - and at speeds that exceed Mach 6.
Apparently the smoke from the gun isn't actually smoke, it's the result of all the air around the projectile being vaporized. Tell you what -- you give me $10 and I'll stand in front of it and see if I can stop the projectile with my belly like an old fashioned strongman and a cannonball. "I'll give you $100." Deal. "Then take it back when you're dead." Money's no good in hell anyways.
Keep going for the demo, but skip to 0:50 if you're impatient.
Thanks to Gabe, who agrees a railgun's greatest weakness is requiring electricity to operate.