Even with AR500, thickness of the target matters. If you're shooting stuff that penetrates well, opt for thicker targets.
Also - that armor piercing 5.56 ammo? Don't shoot it at targets you value. I've had it create divots in mine, while 300 gr FMJ .375 H&H ammo didn't do anything except flake the paint.
My friend Troy runs Precision Tactical here in Wenatchee and builds superb steel targets: