It's not black powder, but here's a good lesson in what can happen when shooting at a piece of steel. This is a video of aguyshooting a .50 BMG at a steel target. The bullet bounces back, skips off the ground in front of him, then knocks off his ear muffs. Scary! Don't shoot at hard targets unless they are properly angled backstops.

http://www.youtube.com/watch?v=0ABGIJwiGBc