What Im doing is making some hanging gongs out of some scrap bar steel we have at work. I choose some 4" diameter steel just want to know how thick to make it so that my .308 won't turn it into instant swiss cheese. Some more info this will be hanging at 300 yards and I will be shooting 168gr FGMM and 175gr .308 at it. thanks for any help and yeah I tried searching but I suck with those things.