49North, If you slug the muzzle of your rifle, you will probably find that it is smaller at the muzzle than it is at the breech. I have a Metford barrel in #2 Musket that has a groove of .469" and bore of .455". At the muzzle, it is .450". Think of it as a long funnel. This is what Metford segmental rifling looks like. Is that what you have? Could it be a gain twist?