Don't waste your time using anything but AR500 or better. 1/2" AR500 will do great just don't do any close up stuff. And, I'd also agree that shooting steel under 300 yards or more is not accomplishing much unless you're using a low powered round.
This is the back side of 5/8" mild steel and the two holes are from a 200 grain Wildcat out of a 7mm AM at 510 yards. I shot the same plate a few days later from 825 yards and it holed it again and ripped the plate off of one chain and wrapped it around one of the support legs. Looked cool through my scope but my friend wasn't overly happy about his plate being ventilated and ripped loose before he even got to hit it once.