Beware the dangers of CHEAP AMMO!
If you shoot enough cheap ammo, it'll happen to you. The rifle is just ticking along, then BAM! The bolt is stuck forward and the magazine blows down out of the rifle. Looking up into the mag. well, you see a crack in the bottom of the bolt carrier. You've just had a case head separation. If you are shooting reloads or surplus ammo, you're out of luck. If you have factory ammo, and the box the shells came in, you can write a letter or call the factory, and they will tell you what to do. We have seen case head failures from overloaded ammo, and from bad brass. There is almost no difference in the result, though. When the case head fails in an AR, the gases flow back into the action. They usually bend or break the extractor, flow along the extractor slot, and crack or break the bottom pad of the bolt carrier. The gases vent out mostly through the mag well, usually wrecking the magazine on the way out. Sometimes the bolt cracks, sometimes it doesn't. In extreme cases, it can crack or break the barrel extension. Many times the bolt catch will break off and the upper receiver will crack by the ejection port. Usually the lower will survive, but sometimes they crack somewhere at the top of the mag well, usually in the front. The front half of the case will remain stuck in the chamber. This is a sure sign of an ammunition failure. If the rifle had failed, the bolt lugs would all be sheared off, the stock would be blown off, the gun would probably be blown in half. We've never seen it happen, and we hope we never do.