Re: steel targets
Don't bother with anything but AR-500.
If you use AR-500 steel, you can use 3/8" plates and they'll do just fine 100yds and out with 223 and 308. Magnums will peck it up pretty good inside of 200yds, but who cares, you just get another one.
A good way to hang them is to bolt the plate to a piece of conveyer belt strapping and bolt the strapping to a saw horse. It'll swing nicely giving you feedback that you hit it.