tube gauge
Very nice work. For those who don't have the skill and/or equipment to design such a gauge there is a bore gaouge made to meaure the inside of shotgun barrels. It has three ball sensors that when inserted into the barrle measures where the barrel "choke"(restriction) starts and ends as well as the interior diameter of the barrel. Butted sections and "choke" are pretty much the same, a tapering internal restriction in a round tube.