A gun device includes a housing having a gas supplying passage communicating with a gas storage chamber of the housing for coupling to a pressurized gas supplying source, a feed tube for feeding a bullet into the housing. The pressurized gas supplied into the gas storage chamber of the housing may all be used to fire the bullet. In addition, the pressurized gas supplied into a manifold of the housing may be used to move a cylinder and a piston and may also be recycled and used to fire the bullet such that the bullet may be fired or propelled with a greater quantity of the pressurized gas and thus may be fired or propelled for a longer distance, for economizing the pressurized gas.