I make targets from 3/8 steel, that I get from cutting off new dump truck, frames, I build. The stuff is not armor quality, but it is very hard. Drill bits just hate it
Maybe check with a local scrap yard or bussiness, that assemble dump trucks. Im not talking pick-up style dumps,but straight framed trucks, will work. The 3/8 will stop any auto loader round, for high power rifles, I double it up. I made a plate rack, that I have to manualy set back up, good exersize, but a PIA. I like the steel B27 style shillouet, I made the best. It has a 3" bull in the center that is hinged, and auto resets. Heres a link to a pic of my homemade shooting range.