Bought an AR off Gunbroker earlier this week. Lower is marked Rock River, but I can find no marks at all on the upper. It was sold from a pawn shop, as a 5.56mm. Tonight I decided to try it out. Steel case wouldn't go into battery, so I tried a brass case and it did. I ejected the round and found that it had developed an extra shoulder. (Shown in pic, beside normal round) Upon inspection, the bore looks slightly larger than a 5.56, and 223 ammo fits high and loose in the chamber? 6.5? 7.62? Where would I find a mark? I haven't removed the quad rail, but I did look in the slots with a light and saw no markings anywhere on the barrel. ?????????????????