When I started reloading my mother taught me how to read a micrometer. She had worked in a plant that made surgical wire during the war (WW2). I later went to a Tech School for machine shop and took a course in Precision Measuring Instruments. While I was in school, I worked nights at a machine shop. Later yet I went into the Navy and went to Machinist Mate "A" school and one part was precision instruments. I breezed through what we had to measure and when I took the problems to the instructor, he said that I needed work on my measurements. Most of the guys were using the micrometers like they were calibrated C clamps. I asked the instructor when the last time his instruments were calibrated. I got no answer.