"The brass base may or may not give you the right size for a chamber gage."
Agreed, my method is subject to some slop. But I have to wonder just where the bucks stops - I would bet that the Galazan's chmaber gauges are not perfect, nor are all shotguns machinined to the same tolerance. Certainly we've seen that to be the case with bore diameter, so I would not be surprised to see some variation in chamber diameter.
It would be easier if there were a definite shoulder, or register, to the end of the chamber but I see mostly guns with tapers which are much harder to tell where they actually begin.
