Very True Mike, & a good point. The chart I posted was from the proof houses & indicates the "Correct" size of a bore, based on a pure lead, perfectly spherical ball that will exactly fit. It certainly was not meant to imply that every 12 gauge, for instance, that has ever been built left the factory with exactly a .729" bore diameter.
For many years the proof housed measured the bores with a series of plug gages. The barrel was marked according to the largest gage which would enter the bore to the prescribed depth. This then denoted a "Range" for the size & not an exact dimension. 13 = .710" diameter thus a 13/1 = .719. A barrel marked 13/1 at proof thus meant it would accept the .719" plug but would not accept the .729" one. It could have been anywhere in between.
The point is though all of these other sizes were named for their "Nominal" bore size from which their chamber sizes were developed. The bore sizes themselves hark back to muzzleloader times. When breech loaders were developed the guns were no re-named for their chamber size but kept their name according to their nominal bore size. "IF" they had been renamed according to their chamber the 12 gauge would have become approximately a 9 gauge (.8025") which would have fallen about mid chamber.
As stated earlier 12 mm falls within the chamber diameter of the .410. 36 gauge (.506") is even well beyond that, Neither is an indicator of the actual size of the .410's (67.49 gauge) bore.
