An actuator housing (20) for use on a pneumatic nailer (10), the actuator housing comprising a casing (22) defining a cartridge insertion opening (24), and a selectively activated actuator cartridge (50) which is inserted in the casing through the cartridge insertion opening. The actuator housing further comprises a releasable second fastening means (30, 69) which allows releasable extraction of the actuator cartridge from the casing. The actuator cartridge comprises an actuator head (54) for sealing engagement with the cartridge insertion opening of the casing and extending outwardly of the casing. The actuator cartridge also comprises a piston assembly (58) received within the casing, the piston assembly comprising a cylinder (100) defining a cylinder chamber (101) therein and a plunger (120) movably mounted within the cylinder chamber. The actuator cartridge further comprises first fastening means (71, 104) to fasten the actuator head to the cylinder of the piston assembly. Upon selective activation of the actuator cartridge, the plunger is actuated along the cylinder chamber.