Sounds foolish to me, even if the rechamber would clean up the existing threaded portion of the barrel, even start load pressures of any .50 cal pistol chambering would be at least double or more the pressure of a BP load, the frame wouldn't be strong enough for the breech thrust with that large a case head and the barrel wouldn't likely handle the pressure, Huntsman barrels are made of 1137 mild steel, same as shotgun barrels.
Tim