I literally just walked back in from shooting a bunch of .22lr at a gong I made by welding up a big heavy disc blade - but otherwise, hard no. Make everything from AR500, nothing less. My disc blade target is designed to always rest at an angle so that every bullet deflects down, and is never hit with anything bigger than .22lr. Ever. I've honestly considered removing that target altogether as the kids get older just so they are never tempted to shoot it with anything bigger than .22lr (I've taught them not to, but I was a kid once...).
You're gonna spend a ton of money constructing a range. You have to buy land and have equipment building berms, clearing openings, and so on. Saving a few dollars per target or a few hundred dollars total, by risking mild steel targets, isn't worth it at all. The fear is having a round leave a divot then having some other round hitting that divot later and creating a reverse-angle richochet. I did it once - with a .45acp - and the bullet came back and hit the porch about 3' above my head and maybe halfway between where my dad and I were standing (maybe 4' apart). Didn't hurt anything but it put an end to me shooting mild steel.
