Clearly a case failure, right where you don't want it.
This is another possible cause: Brass embrittlement due to NOx out-gassing out of deteriorated gunpowder. That is a 2007 LC case, just what is its past history? How did you get it, and was it loaded, or a case from pulled ammunition?
I was pulling targets on a 300 yard rapid fire, when two of the targets called "Insufficient", a bunch of gabbling came down the radio, and we were told a rifle had blown up on the line and there would not be an alibi string.
When I arrived on the firing line, the shooting relay was still packing up, and the Match Director and John were arguing. John was hollering he did not produce bad ammunition and waving his arms. Well, something fun had happened!
There were two new shooters on the firing line, they both had AR15's loaned to them by John, and were shooting ammunition provided by John. And by a remarkable coincidence, they both had same malfunction, in their AR's, on the same rapid fire relay. Both of their AR's blew out their magazines.
this is one of blow out cases:
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the other case was tossed way out in front of the firing line by a disgusted Match Director. But according to the two shooters, it had failed similiarly to the one I picked up. The two junior shooters were not hurt, given new magazines, they were able to continue shooting with the same rifles.
At the end of the match I found the brass was either pull down, or once fired military surplus purchased from Scharch. Might have been pull down. And when I passed the case around, another shooter there said he had similar failures with 5.56 brass from Scharch.
This is a 1988 LC round, which I pulled the bullet
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here are some cases loaded twenty or so years previous, and the gunpowder outgassed enough NOx to cause a lot of corrosion.
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nice picture of factory ammunition
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Pull down brass got on the market primarily due to the fact the gunpowder had deteriorated to the point, that it was unsafe to fire, or unsafe to store. Some ammunition specialist inspected the lot, and determined based on his written procedures, to scrape that ammunition. Civilians buy the stuff, thinking it is "day old bread", only to find the occasional nasty case failure.
I don't know how to tell the difference between surplus brass whose interior has been embrittled, and that which is still good. I understood Scharch tumbled and polished the 5.56 brass they were selling. So even if you have X Ray vision, the green will be gone. This will always be a risk, and since gunpowder deterioration is something the vast majority of the shooting public does not want to hear, that we are bombarded with information in the in print press telling us gunpowder is immortal, and no one makes money telling us what not to buy, this real cause of case failures is largely unknown.