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I would agree with other posters. With the bulge all the way around the base of the cartridge one of two things my have happened... Please note that I am not questioning but just making an observation.
One the gun was not in battery when discharged. As one poster stated it is not easy to fire an AR out of battery but it can happen.
Second, the gun came out of battery before that pressure dropped significantly in the barrel (a timing issue). This "could" happen if too light a buffer was used or a lighter than normal buffer spring was used. Also hammer springs delay the bolt unlocking, if this is lighter than stock it could contribute to the problem... What happens is the lighter springs fail to delay the bolt movement until the pressure subsides to a safe level. The bolt starts to move prematurely and starts to extract the round before the pressure has adequately dropped in the barrel, as this happens a portion of the case is left unsupported while pressures are still near peak levels, the extractor is working the brass abnormally hard since the case has not fully retracted from the chamber walls so it is putting undo stress on the case, all this comes together in a catastophic case failure. When this happens it has all the signs of the bolt not being fully closed when the round was fired.
Since you seem pretty confident the gun was in battery when fired I would opt for door #2.
Glad you were not hurt and the gun did not suffer more severe damage.
Just one old guys take on the issue......
DonT
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