If your AR is for serious work, and it won't run steel case, you may want to rethink your AR.
I'd be more concerned about the bi-metal bullet jackets than the steel cases.
http://www.luckygunner.com/labs/brass-vs-steel-cased-ammo/
I did a lot of research on this particular topic. Basically what it boils down to is the coating they put on steel casings to prevent rust. That varnish or whatever it is will melt off the case and onto the chamber of the gun. If a new round is in the chambered and gun is allowed to cool, the varnish will essentially glue that case in.