Shotgunjones repeats the method I have used for years to measure a .410 bore. Tighten the small hole gauge behind the choke and then drive it out the breech, then measure with a micrometer. Bill Schodlatz mentions the contact area, which is a legitimate caution. However, my Starrett Vernier caliper has knife edge contacts. More investigation to follow. I did not measure the bore with the small hole gauge. However, I did notice that no poster has specified the bore diameter of a Superposed .410, which is what I would like to know, from Browning literature or Schwing.
