Abstract: A cartridge adapter for a firearm including a chamber, a barrel and a center firing pin, the cartridge adapter including a body portion with an axial passage including a first passage section extending from the free end of a smaller cylindrical outer section, a second passage section extending from the free end of a larger cylindrical outer section, a shoulder section adjacent the internal end of the second passage section, a third passage section slightly larger than the first passage section extending from the shoulder section toward the first passage section, a fourth tapered passage section joining adjacent ends of the first and third passage sections, a firing assembly disposed within and having a diameter and a length substantially the same as that of the second passage section, the firing assembly including a breech block member and a firing pin extension member, the breech block member including a throughbore extending from a central point on the free end of the breech block member to a peripheral poi