AR500 should definitely be more than enough to stand up to most anything you throw at it when used right. Mild steel, or generic steel plate (A36) , is far too soft and will promptly become Swiss cheese. Even up in the 1" thickness range. "AR" stands for abrasion resistant, the following number is the Brinell hardness. So the higher the number the harder the material. I have several gongs from 3/8" 450 brinell steel that have soaked up thousands of hits from anything up to 300wm and 45/70 as close as 100yds. they have some very minor dimpling but are just as good as the day I brought them home. The ticket is to hang them. If you line up straight and mount the plate solid, you can punch a hole in even AR550. Let it swing and AR plate should last a very long time
