So far, as much as I can gather, the minimum breech diameter of the barrel of a rifle should be the width of the base of the cartridge multiplied by 1.8. But this does not seem to be the case with shotguns, as black powder fowler barrels of .62 caliber/20 bore seem to have walls that are only 1/3rd the thickness of the internal diameter of the breech. My own black powder rifle has a breech at around 1.5 to 2xs the diameter of the bore. The barrel is not a true cylinder, but octagon in shape. So this leaves me with mixed messages. What is the minimum safe outside diameter of the breech end of a modern smokeless shotgun barrel in relation to the bore diameter?