I just got around to reading some of Joe Carlos' articles on accurizing the AR platform, as linked by
bamban in post #11 of this thread. His position as an armorer for a High Power Rifle Team should qualify him as an expert on the topic of accurizing AR's. One of the inconsistency factors in the AR design that he points out is that the rear of the BCG isn't precisely located when in battery, and that influences how the bolt is aligned with the chamber. Specifically that it can make the face of the bolt not square to the chamber.
Unless I'm not understanding what you're asking here?
That is just one of Mr. Carlos' articles, can search out his considerable list of articles on accurizing the AR on that web site.