Recently bought a Manson gauge, to measure wall thickness in a 20 gauge Birmingham boxlock. In order to have some assurance that I'm using it correctly, and getting accurate results, I intend to practice on a junked barrel the local gun shop owner was kind enough to give me. My plan is to take measurements in the horizontal position. and then cut the junk barrel with a Metabo cut off tool, either wedges cut out opposite each measuring point, or a strip cut out along the whole length of the barrel, thereby gaining access for a machinist friend to use his micrometer to take direct measurements. And then compare those with the measurements I got with the Manson gauge. Anyone have any ideas on how to improve on this procedure?