Steel. They're called "witness plates" and are used in competitive handgun shooting, because a handgun round will create a bearing of witness to the plate being hit; i.e. marking of some sort. High-powered rifle rounds have far too much kinetic energy to use these things at the short distance he was using them at...a military sniper on a .50 can explode a human head at several thousand yards, and this dipstick had functionally the same bullet set up to strike a steel plate 100 yards downrange.

ETA again: I met a Marine sniper who used a .50 in the opening days of Gulf War, Part One. His orders were to take out satellite dishes. A .50 into the center of most satellite dishes is enough to unmount the dish from whatever it's bolted to. Bye bye, enemy comms. And I say again: steel at 100 yards.