The toughest stuff I have found is called T-1. Any steel supplier knows what that is. I made some targets out of 1/2 " thick T-1 and thought I could blow some holes with a 7mm mag without drilling. At 20m, it put a crater about 1/4" deep in my target and I had to another 15m to get the thing back. If you are shooting "Ball ammo"at it, you might want to do a little expermenting with the angle of the plate and allow the bottom end of the plate to move when it gets hit.
AR steel is like taking pot luck, it is stuff that didn't make the grade they were shooting for at the mill. I'm not quite sure what the stuff is.
Hope that helps.