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If you are anywhere near St. Louis MO, let me know and I will teach you how to use a mic to get repeatable measurements to the tenth. Is it hard? No. Can anyone do it with a little practice…yes. Using pin gages to measure holes can get hairy when you get down to the tenths. You have to develop and have “the feel” when that pin gage slides in. Ideally, when it comes to sizing bullets, it is best to run a slug through the die and measure the slug…and preferably the slug should be of the alloy you intend to use.
Lapping an already hardened die with proper laps and technique on a lathe is not difficult. It can be kept both round and concentric. By hand or by drill does make things a bit more difficult.
As far as buying a mic, I am not rich by any means, but my advice would be to buy the best one you can afford. If you buy a cheap one that is not well made you have gained nothing over using a standard caliper.
I have 8 or 9 sets of mics, of varying sizes and types. They all get used regularly (not for bullet casting stuff). I think someone mentioned Ebay???? I would look for a good used Starrett, Mitutoyo, or Brown and Sharpes micrometer. They are on their all the time. I would recommend making sure the micrometers come with 1.) The case 2.) The adjusting wrench