You proved that your load will destroy an AR500 target at 100 yards.
Velocity is the culprit that damages the targets. Simply move back until the target is not damaged.
Allowing the target to move will also allow it to adsorb the energy, you only have the bolt to hole clearance in this case, mounting it on something that swings will help a lot.
AR500 comes heat treated from the mill. It is heat treated in continuous sheets over 10 feet wide, then cut into sheets and sent to the fabricators in the hardened condition. The target manufacturer does NO heat treating.
Method of cutting, ie, plasma, laser, waterjet has very little to do with the heat treat. The affected zone is only about 1/32".
All targets can crack when hit near the edge,,,, primarily because there is NO support at the outside edge.
Target bullets , or full metal jacket, both act the same on a target, that .030 to .060 soft copper jacket disintegrates when it hits the hardened steel.
I was an AR500 target manufacturer, and I know that any body can ruin an AR500 target.
Police departments that shoot 223 at AR500 at close range regularly turn them into cratered bowls.
Range operators have to replace target regularly.
Catching bullets is hard work, give your targets a break,,,,, velocity is the culprit,,,,,,,, move back!!!