Abstract: A toy water gun comprising a housing, water tank, nozzle, pump assembly, piston drive assembly, and crank handle. The pump assembly comprises (a) first and second cylinders, each cylinder having a first cylinder end with an opening for receiving a piston end and a second cylinder end with an intake port and a discharge port; (b) a piston having a two piston ends, a piston shaft between the two piston ends, and a drive pin on the piston shaft, each piston end inserted into the first open ends of the first and second cylinders, respectively, and being slidable along the longitudinal axes of the cylinders in sealing relation to the cylinder walls; and (c) a device means for filling the two cylinders with water from the water tank and for discharging water from the two cylinders through the nozzle, wherein the filling and discharging device act in cooperation with the slidable movement of the piston ends. The piston drive assembly comprises a cam arm and a coupling wheel.