I remember shooting a short-barreled AR at Tri-City about 20 years ago. I don't recall if it was a true SBR, a 16" barrel, or one with the "permanently pinned" flash suppressor, but I do believe that it was the loudest small arm I've ever heard, painfully loud even with high-quality hearing protection (its report was significantly louder than my A2's). I can't imagine busting a cap on that one indoors...I always double up on hearing protection when going to a range, indoors or outdoors. Even then, short-barrelled rifles with muzzle brakes can cause some pain.