I recently watched a special about these kinds of weapons on the Discovery Channel and there are a few clarifications I'd like to make.

First of all, the slugs that are fired are usually made of a heavy metal, like Tungsten.  Sometimes, depleted Uranium is used (think of where else it's used-- the A-10's GAU-8A Gatling Gun).  Also, a metal casing is used around the slug, so that once it clears the rails, the drag in the air splits the casing off, revealing the smooth slug.  From what they said, the slug can travel at speeds in excess of 10 times the speed of sound