The present disclosure relates to kits and associated methods for converting a semi-automatic firearm into a bolt single action firearm, as well as the converted firearm itself. The kit includes a housing (e.g. upper receiver) that accommodates a bolt carrier and a charging handle and a carrier key, if desired. The housing has ends which mount to the existing lower receiver of the semi-automatic firearm to convert the semi-automatic firearm into a manual bolt-action firearm.