I have some ARs with BCA uppers and the only problem I've ever had was with their 12.7x42 upper seeming to be undergassed. That upper was returned to BCA and I'm currently awaiting the availability of their latest version (mainly with a larger gas port now, from what I can tell).I’ve never seen a properly built upper need any break in ever. These are AR15s not 1911s. FWIW Outside of a Les Baer I’ve never seen a properly built 1911 need breaking in and Les Baer advertises that you will need to break it in based on their tolerances.
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Although my Les Baer Boss .45 was (and still is) tight as hell, it has never missed a beat, functioning at 100% from round one.
edited to add...oops, I see I already replied to this thread last year
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