Virtually every AR15 I have ever touched has that annoying looseness between the lower and upper. Found a great article addressing this issue and the explanation put the problem as plus mill spec and minus mill spec. That is if the upper leans more to one side of the acceptable tolerance and the matching lower leans in the opposite direction for tolerance you can end up with sloppy fit (not tight).

Would that suggest we should buy matched upper and lowers as a combination only?


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