A pump action firearm includes a barrel and a chassis defining a magazine well for receiving a magazine. The magazine well extends longitudinally parallel to the barrel and contains a plurality of cartridges arranged in stacked relationship with each cartridge having its longitudinal axis extending substantially transversely to a longitudinal axis of the barrel. A pump member displaces the barrel forwardly and rearwardly. A carrier mechanism is linked to the pump member and, during pumping of the pump member, moves an end cartridge contained in the magazine from its stacked position to an operative position in alignment with the longitudinal axis of the barrel.