What was the manufacturer, the round count and exactly how does mild steel "tear up" tool steel?
In the case I saw, the rifle was a CMMG, and the owner had been shooting Wolf exclusively for "about thirty boxes."
I'm not sure the steel-on-steel contact was the only
cause, as the bolt face was encrusted with a level of scrunge I can only describe as "epic." I'd hazard a guess that it was a combination of the two, but I've never seen it on a rifle fed brass-cased ammunition.