T Cole,
I have been shooting AR400 3/8" and 1/2" with a 22 Swift, .260, .308, and 300WM from 300 to 1200 almost every weekend for several months. No damage to steel. When I first made them I was breaking the chain I used to hold a 6" round plate to the backside of the front plate that had a 5" hole in it, 300WM with 190r SMK. I will look for some pics of plates I played with that were mild steel.
Steve