1/2" thick mild steel is no match for .308 or .223 at closer ranges after you hit it a few times.
If you want a thinner plate that will withstand high-power rifle fire, have a machine shop cut you a plate of 4140 steel, hardened to 40 Rockwell C. I believe my dad's are only 3/8" thick, yet withstand .30-06, .308, .257, .270, and 7mm at 100 yards. The hotter rounds will cause perhaps a 1-2mm dent, but that is all. It *will* cost you, however. I think a 12" diameter plate is around $100 - but it will last forever.
-z