In 1910, the nominal size for a 12 bore shotgun, is .725", .727" and .729". Different sources differ in specs. Today, with special boring to relieve recoil, etc, back boring, whatever, many run around .735" to .740".
: 12 bore rifles can run from about .730" to .750" on the grooves depending on depth and orginal bore size. This is how I understand the situation. There were no SAMMI specs., although good gun makers were very capable of making all their high-grade guns to 'exact' sizes if ordered so. Most good quality guns came with ball or bullet moulds for that particular gun and some even had swages for exact fit of bullets for the barrel.
: With damascus barrels, sometimes there would be irregualrities or inclusion, pits in the weld or other annomlies which required additonal boring, which enlarged the bore some to considerably. I am not aware of 11 bore rifles - they were either 12+ or 10 bore. Seems to me, some guns had marks on the table or bottoms of the slightly oversize 12 bore barrels with a + sign along with the 12 to show whether the bore was larger than standard.
: Were the bores of RB's gun .727" or was the groove diameter .729"? .006", which is .003" per side is considerably oversize for a .729" groove diameter. I suspect the gun would prefer a smaller ball, perhaps .730" at most.
; If the ball is soft, it will reduce to fit, but will raise pressure and change harmonics. My best accuracy from smoothbores has been with balls of correct size to the bore.
: Look to the cape guns and paradox rifles, with a ball or quite substancial slug traveling over 1,000fps when hitting the rifled choke at the end of the barrel. Swaging the ball into the rifling at the breech upon inital firing would be less hard on the gun, I am sure, certainly less hard on the soldered together barrels. I would size (swage) the balls or use groove diameter balls.
: When I cast up balls for my friends to use in their pump action duck guns, we made sure they could push the undersized balls and patches through their chokes easily. This led to impressive groups at 50 yards (considering the double barrel-beads), and more than one dead moose, shot from the duck blind.