Those barrel stubs with the reamer ran in are called "neck Checkers". The term came from wildcatters and benchrest shooters necking down cases from other cartridges other than the formed case. The brass gets thicker when you neck it down, if the case would drop in the barrel stubb, then the round was safe to shoot in the rifle, in as far as neck Outside Diameter was concerned. The added benefit came when someone realized that you could also check the Bullet seating depth. Milling away 1/4 of the barrel stubb along the bore axis reveals the leade angle, amount of throat, and distance between the front of the case and the point where the leade angle starts. You can not measure these distances, but you can sure see them.
