bolt not locking back on last shot

[babaganoush]

I had an issue with mine that, in addition to a FTF would often fail to lock on empty. While installing the barrel, I'd apparently overtightened the clamshell holder, and had somehow deformed the upper. It prevented any magazines from seating correctly and it, too, would always lock back when manually cycling the bolt. Mine didn't fail to lock on empty as consistently as Terry's, but it was one of those "drive you nuts" problems. There were definite marks on all the metal magazines where they were contacting the upper receiver, which is what finally lead me to check. Swapped the upper and made all the above issues go away.