So your bore is .760 based on the muzzle measurements. A 10 bore has a .775 bore as a standard so your is slightly tighter. Often 10 we’re bored to 11 bore diameter and 12’s to 13 bore. Your might have always been a 10 with a bore that was 12 which is now bigger because it got cleaned up several times to remove pitting.
I had a hammer gun that was proved as a 12, at .729 presumably, that had been hogged out to .783. The weird thing was that it was really clean looking on the outside and even had a little case color left.
