I was talking with a reloader a few weeks ago about "Acceptable Tolerances." I was saying My original dial Caliper [for case length trimming] only measured to 1/100 of an inch, and weighing powder charges was on scales marked in 1/10 grain, but the scale was listed as accurate to within +/- 2/10 grain.(that meant the lightest to heaviest could very by 4/10 of a grain!)
Now I insist either on "NO Trim" cases (for my AR's) or the other extreme I trim to 5/10000" (for my 1000 yard bolt actions)
I have very good balance single and 3 beam scales the are within .05 to .10 grain, as checked by certified check weights and two good digital scales. However I got a notice from Sinclair International that they offer a scale that is 1/100 grain accurate (only 4 or 5 times the price)
In that same time frame (late 1970's), I checked match bullet weight consistency, Hornady Match 168 gr HPBT varied by up to 10%, back then Sierra was about 4%. Now days, Sierra very about .5% ON SECONDS!
In my assortment of tools I have a Starrett Micrometer that runs from 1" to 2". It was given to me by my oldest son because it only measured to .001" (1/1000") and he needed one for .0001" (1/10000") Is he a machinist? NO, he is an Airframe Helicopter Mechanic! and has to verify all materials used for repairs! (I guess that's why he makes the big money!)
Ivan