Something we also run into, was very early slug guns & ctg. guns are measured or stamped for the bore size, which is the norm. for muzzleloading guns, not for the groove diameter.

When shooting slugs, my 14 bore muzzleloading rifle is actually a loose 13 with it's .690" bore and .714" groove diaemeter. My 16 bore Husky SxS is a 16 x 13 bore as the left barrel is 16 bore @ .662", while the right has a 15 size bore @.675" and 13 size groove @ .703" It can be confusing at times even without the re-bore after initial proof.

My chart shows 11 bore as .751", 12 @ .730", 13 @ .710", 14 @ .693", 15 @ .677" and 16 @ .663".