^ agreed.
If you're talking about absolute measurement of the longest range, then sure. If you need a length of 6.000", even an expensive pair of calipers may be off by 0.005", I will suppose.
But if you are looking at smaller measurements, this is simply not the case. I used regular calipers to gauge a distance of 0.401" with no problem. At that distance, you have less than 1/12th the potential error. I'm sure my $10.00 calipers are good enough to get within +- 1 mil or so at that range. And if needing a relative measurement, comparing to, say a particular bullet as my reference, and I just want the part to be at some particular difference in comparison, I am confident they are under 1 mil, maximum.
When the calipers go on the bullet and show .401, then they go on the plug and show .401, I know I'm within a half a mil, not 5 mils. Or maybe just a tiny hair more, if I'm extremely unlucky. I know this because the calipers register a nominal half a mil, and they read the same thing no matter how many times I put them on the object. If the bullets were any much more than .40125, they would register .401 1/2. If the plug were any much smaller than .40075, then it would register .400 1/2. That's the worst case scenario. And none of that even matters, because the end result of neck tension is what is important, and you can't measure that with calipers.
I'm not a machinist, but I use calipers quite often. And usually for measuring very small things, or very small relative differences. I have not lacked for the need of a large absolute measurement in my usage in reloading or otherwise.
You might forget how to do things with subpar tools. Most folks don't mill and machine large steel objects to other people's specifications, where going under by a couple mils will be costly. If we're doing it at all, where a mil or two matters, it's to make it fit something we already have in our hands to compare against.