Top hinge them so the bottom swings away when hit. That not only absorbs impact and helps prevent burn-through but also causes ricochets to hit the ground below the plates. I have a series of 1/2 inch plates that take a dent from 5.56NATO at 100 yards, the 7.62X39 hurts 'em prety badly, 7.62NATO penetrates almost completely, 30-06 goes straight through. I have no idea what grade of steel it is, but they are heavy.