Thank you for your reply Richard. I am very familiar with the origins of the gauge system for bore size. What I meant by "modern", is when did guns become standardized as to where a 12 gauge gun had a .729 diameter bore. This is a fairly modern occurrence, I would estimate this happened sometime in the mid 19th century. For example the British Board of Ordinance documents that a musket's "caliber" was not the bore size, but the size ball it shot. A Brown Bess musket had a actual bore size of around .760-.800 but fired a ball of around .690. It's caliber was referred to as being 13 balls to the pound, not 10 or 9. The same for the 42 pound cannon example you gave. For arguments sake say a 42 pound cannon ball measures 6". I will bet you a Coke that the actual bore diameter of the cannon is larger than 6", so why isn't it called a 41 1/2 pounder or a 41 pounder, by "modern" standards?
Today things are backwards from those early definitions. If you can find ball slugs for your 12 gauge, they will not measure .729, but somewhere around .690-.700. Why don't we call the gun a 13 gauge? It is because more recent standards have determined that a 12 gauge should have a bore size of .729 regardless of what size ball it shoots. I might argue that a Parker with a .750 bore is a more true 12 gauge because it can shoot a full sized .729 ball.
I think if there was a wide spread data collection of the bore sizes of the guns in the serial number range Austin gave (7,000-70,000) we would be shocked at the thousands of "11 gauge" guns that are floating around out there, if we went by bore size alone. Your reference to chamber dimensions may be the only way to accurately determine what the gun was originally classified as (if the Parker records for that specific gun are incomplete). Throw in the "A" and "B" chamberings and you get to a whole new level of headaches. I thank you for trying to add some clarity to this subject.
Do any of you know if there has been published data made available of full hull/chamber dimensions of the A and B versions of each gauge. This might go a long way as to clarifying this topic.
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