Originally Posted by
AKDoug
Your lower is right on the bottom edge of the spec. The lower receiver blueprints call for .500" +0.000/-.004 It is likely that it will be problematic with many uppers. I've yet to find a blueprint for uppers so I can figure out what their specs are. (ETA: I have found upper prints, but none of them have the dimension for the mating surfaces between the upper and lower) I would imagine that there is some point within the tolerances that a upper on the big side of spec won't fit in a lower that is on the small side of spec.