Forrest, I have shot both without any real issue. The only thing I can say is that brass is more forgiving on the reliability on a Mil-Spec AR. There is less case expansion (read sealing) during firing and you will definitely dirty up your rig twice as fast. After a few hundred rounds of steel case you will likely need to clean out your chamber throat to reliably start firing brass cased ammo. You can shoot both but I don’t recommend mixing during a firing session.