Quote Originally Posted by Joelski View Post

This, plus the fact the majority of people who throw around the term "Mil-Spec", really mean to say "We took measurements and reverse-engineered the Colt piece, plus or minus." Thus, the origin of tolerance stacking.

Another important facet of tolerance is end-play, or "slop". I have two really high quality, expensive components that barely fit together, let alone reciprocate properly because interpretation of dimensions and tolerances isn't the same as known, good measurements backed up by gazillions of pallets of rounds.

Along with FN, I'd venture that LMT has also been trusted at some point to peek at the TDP.
SDI was, as well, and actually built some contract guns. That said, they are no more.