Has anyone seen muzzleloaders marked as to gauge? I have seen a couple that are marked "13" on the underside of the breach near the forend. The "13" appears next to the provisional and final proof marks in one case involving Birmingham tubes. Because of the size of the barrels, I have guessed that the "13" may mean that it is a 13 gauge muzzleloader. Is that right? If not, what does the 13 refer to?

Rich