Abstract: A conversion unit for a firearm including a barrel assembly which replaces a standard shotgun barrel used on the type of shotguns that utilize quick change barrels which enables an existing shotgun to be converted into a muzzle loading rifle. The conversion is accomplished without affecting normal operation of the shotgun thereby enabling the muzzle loading conversion unit of this invention to be removed and replaced by the previously removed shotgun barrel. The muzzle loading conversion unit includes a barrel that has the same general outside dimensions as the standard replacement shotgun barrel with the conversion barrel including rifling and adjustable sights with the breech end being provided with a plug that is chambered to accept a standard shotshell primer with the breech plug also having a spring loaded extractor which will extract the spent shotshell primer a short distance.