.556 will dimple steel if you shoot it too close. The more dimples you get, the more chance you have of bullet splatter going in unwanted directions. If you shoot the steel only with rifles from distance, you wouldn't worry about that. I shoot mine with pistols a lot, so I do not let people shoot it with anything other than frangibles from a rifle with supersonic ammo.
Some people that used my range in the past did not listen to me, and you can definitely tell.
Not hawking their wares, although I do have a bunch of their plates, but there is great info here.....https://shootingtargets7.com/pages/target-selection
One thing that MAY help prevent damage from "too close" rifle rounds is to put a pretty aggressive downward angle on the plates...