You can purchase a ball attachment for an ordinary flat anvil micrometer. Pretty sure the one I have has a diameter of .200" & has a collar to center it over the anvil. You can simply run the spindle down against it to verify the reading. Measure a few bbls at points you can each with the micrometer & then with the wall thickness gauge to confirm. If its accurate at one point, then as long as you make certain you don't lket the shaft of the WT gage hit the side of the bbl it will be accurate at any point. This same caveat applies equally even if you cut up a bbl. There is however no real need to go to the trouble of cutting up a bbl, even a useless one, to prove it. You are of course goint to need either the ball mic or attachment either way so nothing extra required.
