"Tolerances." Now, there's one that a lot of people throw around and they completely misunderstand what it means in a factory production setting.
Things made on a production line have dimensions - they are x long, y high, and z deep in common understanding, albeit a CNC machinist may take exception to even that. When making a run of parts, the "tolerance" is measured to a plus or minus as both men and machines are not perfect at continuous reproduction. (Even with their gene pool.) So if the part is 6" long, the engineers have calculated that it is allowed to vary only as much as, say, plus .015" or minus .015", which is a total of .030" tolerance. The people checking the parts literally "tolerate" that much variation in size.
During assembly the engineers also have a tolerance spec for the collection of parts put together, and if you add all those pluses and minuses they limit them. That is a "stack" tolerance, which sometimes forces assembly workers to sort the parts by smaller or larger critical dimensions and to fit the complementary ones to meet that figure and prevent going too far under or over.
Now, for something that has to move, such as a bolt inside a receiver, there is an additional amount of room to accomplish that without excessive friction or sticking. It requires knowing the amount of expansion and contraction a device may undergo due to the ambient temperature and what heat or cooling it may produce. Something which has to perform at 15 degrees F up to 145 degrees F may undergo some amazing dimensional changes. Yet that is a common situation for guns in North America, winter to summer. Yet it is still required to operate, so the engineers put in enough space between parts to allow it. That is known as "clearance."
The issue with a gun made to close tolerances is moot, it's generally considered to be good quality control, but one with tight clearances touted as being the pinnacle of fit and finish? It could likely jam up at extreme temperatures if there isn't enough clearance, even more if contaminated with dirt, mud, or freezing precipitation. This is where a highly rated gun at home in 72 degree dry environment becomes a nightmare at 31 degrees in wet conditions. They can be problematic. And pulling one out of the trunk that sat out in the afternoon sun in August, not so much either. Much less the heat soaking that the cartridges experienced and how much it affects the burn rate of the powder.
Or, a newly made M16 with a outsourced barrel and undersized chamber, firing contaminated powder repurposed from other intents. The guns tolerances were inspected repeatedly, but the working clearance of ammo to chamber was something else, especially when fouled. Tolerances are not clearances, and when it gets discussed in forums it's typically confused. Even worse, it's considered the hallmark of good gunsmithing to reduce clearances to the minimum possible, when the reality is that there better be some and you have to accept it if you want the gun to operate.
Which is better, a 1911 which has a slide that actually shifts from side to side when handling it, but which can hit a man sized target at 50 feet in any environmental conditions, or one with no slack at all, which freezes up in the holster just getting there, or binds in the heat of the desert? I will take the old service .45 every day and leave the cantankerous high cost race gun at home where it belongs. Both were made to the same tolerances, =/- .015", it's the clearances which make all the difference.
How has this misunderstanding developed? 50 years ago we were a nation of producers, and many of us worked in factories. We dealt with numbers, calipers, and dimensions all day long. Now? Very few, and most have no real clue. The bulk of American workers are in service industry, not production, and the problem has grown exponentially worse as our understanding of it has become out of our ken. Like military servicemen, who were so plentiful at one time there was at least one in ten at a family reunion, now it's one in one hundred, and the ratio of folks who actually produce parts and create machines is about the same, now, too. The average American actually doesn't know what they are talking about anymore when it comes to making things or serving our country. Having done both it's one of those things where you just learn to keep quiet about it most of the time. It takes much too long to explain, and all too often the unknowledgeable hearing things they don't fundamentally understand in the first place question what is being said.
Tolerances are not clearances.
. Just kidding. I think I'll just skip this.
