I've assembled a few hundred uppers at my last employer and it does happen. Annoying as hell when it does. Not as bad when it's just a plain GI barrel nut where I could just go to the spare parts and try a few until one works. With some of the rail systems that used their own nuts it was a bitch.
It doesn't mean anything is out of spec, just that both the upper and the nut could both be at the large end of the spec. The plus and minus that every spec has (that's what a tolerance is by the way, the plus or minus of the dimension of a part, not how tight or loose the parts fit together... that's a side effect of extremely loose tolerances)



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