Forrest,
The "modern" definition of Shotgun gauge size in relation to bore diameter is determined by the laws of physics and mathematics and the English system of weights and measures. A sphere of lead that weighs 1/12 of a pound has a diameter of 0.729 inches. It takes eleven lead spheres of 0.751" diameter to weigh exactly one pound. It traces back to the middle ages when cannon bore dimensions were defined by the weight of a lead ball they fired. A full cannon was a 42 pounder and fired a 42 pound cannon ball. A 12 gauge gun that is severely pitted or has been honed could easily have the bore diameter of an 11 or even a 10 gauge size. We know from the records that in the early days (1872 to 1875) Parker made a number of 11-gauge guns and chambered them 11A (.825" head diameter) and others were made for 12A brass or 12 Paper shells (0.810") which is the proper chamber size for 12gauge guns made today, Some of the early 11-ga Parkers were chambered for 10A shells (0.849" head diameter) and have a record of one 11-gauge Parker that was chambered for the Parker 11B shell ( 0.792"). I have also measured a 12-gauge Parker that was chambered for the 11B shell
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