http://news.aol.com/story/_a/navy-tests-high-tech-railgun/20080131154909990001?ncid=NWS00010000000001

The physics behind the basic concept is exceedingly simple, the engineering must be incredible to do it in practice...

A quote from 4 years ago:

Forces inside the gun itself—particularly getting the armature to move easily within the system—are also challenging the designers. “Getting two pieces of metal to slide past each other is pretty hard—we’re getting a lot of damage to the rails,” Beach says.

The electromagnetic railgun’s projectiles will cover 290 miles in six minutes—initially traveling 8,200 feet per second and hitting their target at 5,000 feet per second. Current Navy guns, which shoot powder-ignited explosive shells, have a maximum range of 12 miles and, because they are unguided, are difficult to aim. Though guided missiles, the current long-range alternative for destroyers, can achieve ranges comparable to that of the electromagnetic railgun, their cost and storage problems are what’s driving the efforts to find an alternative. Ships can only carry up to 70 guided missiles and must return to port to restock because the missiles cannot be loaded at sea, whereas railgun projectiles can easily be loaded at sea, and by the hundreds. Also appealing is that the electromagnetic railgun’s missiles do not contain volatile explosives; the weapon does its work with kinetic energy.