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I assemble a modest number of ARs each month. I occasionally see issues with the upper holes being out of spec by a few 1000ths. It has always been the upper when I measure things. I end up milling the upper, front takedown hole, inside side of hole, extending it 4-5 1000ths. The upper then fits perfectly on multiple different lowers.

I’ve seen this with multiple “brands” of uppers but likely all made by the same machine shop under contract. They usually have the same maker mark on them. I have only seen this when using complete uppers a customer specifically wanted. When I build uppers I have never had spec issues (I use Anderson uppers).

[QUOTE="CrackPot, post: 5957593, member: 27498"] I assemble a modest number of ARs each month. I occasionally see issues with the upper holes being out of spec by a few 1000ths. It has always been the upper when I measure things. I end up milling the upper, front takedown hole, inside side of hole, extending it 4-5 1000ths. The upper then fits perfectly on multiple different lowers. I’ve seen this with multiple “brands” of uppers but likely all made by the same machine shop under contract. They usually have the same maker mark on them. I have only seen this when using complete uppers a customer specifically wanted. When I build uppers I have never had spec issues (I use Anderson uppers). [/QUOTE]

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