We used 2/0 chain and grade 8 bolts with grade 8 washers and nuts. The chain mounted on the back makes the plates lean forward, so fragments will be directed down or be weakened if they're driven up. We used AR500 7" circles from qualitytargets and 1/3 size IDPA style tombstones from Wideners.com. They're mounted to frames made of black iron pipe stuck in the ground with spikes.
We used 2/0 chain because we figured if a hydraulic cutter at Lowes can cut through it, so could a bullet. 2/0 is sturdy enough but cheap to replace when it gets hit (rifle bullets cut through them like butter, but pistol bullets knock the zinc plating off)
30-06 and .308 slightly dimples the metal at 50 yards, 5.56 slightly dimples the metal at 50 feet. The edges of the metal are soft because the plasma/laser cutters ruin the temper when they're cut out, there are a few nicks on the edges from rifle bullet near misses. Welding can also soften them, too.