Never personally had issues with the M4 or 5.56 performance, nor do I know many guys in combat arms (and none in the SOF unit I served in from 04-08) who had complaints.
That said, part of that may stem from realistic expectations of performance -- I don't expect my issue long gun to shoot through mud brick walls that soak up .50 cal, Dishka and even occasionally Russian 14.5mm hits. If the leadership/tactical ability wasn't there to put effective fire on those targets that's a training issue, not a tech issue.
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