Thanks Shivan. I wanted to give a quick update on this. First off I want to express my gratitude to AAC, especially Mike Mers @ AAC.
Mike took the LMT upper apart, and inspected the barrel, flashhider, threads, etc.
The response was that the flashhider was on absolutely straight (much to my relief, as I was worried I had borked that process up even though I followed instruction to the letter). Threads were spot on, barrel, etc. The overwhelming theory lines up with what has been said before, that a round may have been poorly seated in the casing, or a weak primer may have caused an under-powered round.
Either way, no baffle strikes, and just a small nick, resulting in a small bulge on the end cap. So this was not a fault of the AAC product, and in Mike's words, "just bad luck". I want to go on record thanking them for still offering to replace the end cap and make it look good as new.
Thanks again to AAC, and I appreciate all the advice I got here on this thread.
"There he goes. One of God's own prototypes. Some kind of high powered mutant never even considered for mass production. Too weird to live, and too rare to die."
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