I've got a Wooley and Sargent double muzzle loading 6 bore myself. Odd gauges in big bore muzzle loaders aren't uncommon. Yours is in really nice shape apparently, mine is a major beater. I bought it years ago just because it was so huge and cool looking. The barrels are too thin to shoot sadly, I'd love to give it a try.

Destry

Out there at the crossroads molding the devil's bullets. - Tom Waits