Action Target has an excellent post explaining what goes in to making a good steel target. They included some slo-mo video of what happens when bullets impact a steel surface and effects of various designs.
Part 1: http://store.actiontarget.com/blog/making-sense-of-steel-targets/
Part 2: http://store.actiontarget.com/blog/making-sense-of-steel-targets-part-2/
From: Action Target
