I agree with all of the above especially going to regular bolts instead of carriage bolts. Your carriage bolt failed at thin part of head.The 'Grade 8' bolts are usually tougher. Maybe try a regular bolt with a washer to help distribute the load better instead of the carriage bolt.
At my club, we use rubber/plastic hangers between the steel plate and the chain that helps eliminate some of the shock to the plate and hangers.
At 200 yards, you should have safety concerns adequately addressed.