I've heard a lot of people say they don't like to shoot steel cased ammo in their AR.
All I can say, is it ain't much of a gun if shooting steel cased ammo breaks it!
Over thousands of rounds the steel casings can round off extractor claws on AR's due to the "pop over" style extraction system.
Not to mention that most steel cased ammo is lacquer coated and causes problems on its own.