Originally Posted By KILLERB6:
Oddly, there seem to be almost as many posters complaining about(too) tight fitting uppers/lowers.
As long as the two parts stay together, they're good to go.
As an interesting aside and perhaps as a testament to the adaptability of the AR beyond what most people feel is prudent to do, there have been the occasional reports of people complaining that their AR is functioning, but doing so unreliably, particularly with regards to feeding problems.
Further examination then reveals that the user has in fact mated an SP1-style "big hole" .370 pivot pin upper with a standard .250 pivot pin lower without any shims or bushings, but simply assumed it was "plug and play."