Why do you think you need a mic? Anyone who shoots and handloads needs calipers, they are much more flexible and user friendly....as far as the tolerance issues I do not think you will be able to see much of a difference with a 3 thou variance on a "skirt" that will expand to the barrel walls upon introduction to heat and pressure....plastic is very flexible in this application and will essentially mold to what it needs to be, especially in the form of a powerbelt skirt.....my 2 cents.