Aight yeah it really reads like praise. Let me clarify further.
Are excessive. Even in the closest running fits listed in the Machinery's Handbook, +/-0.0001" to +/-0.0002 is kinda just mental and there would be no operating difference if the tolerance was +/-0.001", or five to ten times more slack. The holes that these parts go into are 0.005" over the piston/bolt body diameter, and having those +0.002"/-0.000 (unilateral) would prevent any harmful tolerance stacking and maintain part clearance for the close running fit.
A human hair is 0.003" thick on average. The tolerance range for those parts was in the order of the thickness of a human hair spliced TWENTY TIMES IN THICKNESS. Stop the madness. A rifle made with tolerances ten times more loose would operate exactly the same; the difference in the parts on our everyday scale is just too small to change anything. The headspace range for 5.56x45 is (IIRC) around 0.003", so even with looser tolerances, you could probably safely exchange bolts between rifles with no real difference.
You might say "but CNC machines these days have no trouble being that accurate" and I'll reply that even with the highest quality and most accurate CNCs, that isn't so simple for mass production (the machine is fine, but the tool wears, the machine warms up/cools down, tool deflection factors between roughing, finishing, etc). To get parts that consistent all the time all day every day, you're going to need to have more machining operations that take time and money, and you're going to be replacing cutting tip inserts (each over 15 bucks each, it adds up) very often, better measuring equipment, and more qualified personnel.
Knowing how much precision in the manufacturing is required to make something good is a complicated thing, and often marks the line between a good engineer and a lazy one. A lazy one throws ridiculous tolerances around because he can't or won't be bothered to figure out how much is really needed.
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