You didn't say what kind of rifle it was. Several years ago, before PA had a muzzleloader season, my cousin owned a T/C Hawken percussion rifle. When we he learned of the "new" muzzleloader season he contacted T/C and purchased a kit to conver it to flintlock. This kit consisted of a new breech which had the flash hole, a tool which fit over the octagon barrel, and a lock.
We had to soak the breech in kerosene for a few days then clamped the barrel in a vise slid on the tool and with a good amount of elbow grease, unscrewed the old breech.