I'm going to start off by saying that I don't claim to be a machinist, barely claim to be a gunsmith, and certainly have no formal education in either. That said, the idea of measuring threads by Pitch Diameter seems to be a relatively simple concept. So why are there so many people in this particular industry who routinely quote the Major O.D. as an indication of thread fit? I've worked in at least 3 other heavily mechanized industries, and never noticed the confusion until now.