A rifle kit comprises a rifle receiver structure having a central passage configured for having a substantially as-manufactured breech bolt of a pistol mounted therein and has one or more shoulder stock mounting portions configured for having a shoulder stock structure engaged therewith. A substantially as-manufactured breech bolt of the kit is configured by an original equipment manufacturer (OEM) thereof for being mounted on a pistol receiver body configured by the OEM. The rifle barrel is mountable within the central passage of the rifle receiver structure. A chamber end portion of the rifle barrel is configured for engaging a mating portion of the substantially as-manufactured breech bolt. A shoulder stock structure of the kit includes a receiver structure engaging portion configured for allowing the shoulder stock structure to be mounted on the rifle receiver structure exclusively through engagement with the one or more stock mounting portions of the rifle receiver structure.