I only own 4 muzzloader. 2 of which where rusted and pitted when I bought them.

try giving it a bath in hot soapy water. remove the breech plug and the barrel from the stock. once you have it in the water, with your ram rod or cleaning rod start doing a pumping action.
once you done, run as many patches you need to dry the bore.

another thing you can try. you can get a rust remover. apply it to a patch and run it down the bore. than take a 50cal bore brush and start scrubbing. once done, give it a bath in clean hot water to get the chemical (never mix chemicals).

what every you do. get some jb bore paste.