Disclosed is a needle-free injection device for delivering a medicament under pressure from a dosing reservoir through an outlet orifice for administration to an animal or human. The device has a plunger slidably received in the dosing reservoir and movable in a forward direction for expelling the medicament through the orifice. The injection device includes an actuating device and the actuating device includes: (a) A gas tight chamber. (b) A piston and rod assembly slidably received in the chamber and movable between a forward position and a rearward position. (c) A gas charge in the chamber for urging the piston and rod assembly to the forward position. (d) A charging mechanism for moving the piston and rod assembly against the gas charge into the rearward position. The charging mechanism includes a threaded shaft threadingly engaged with a nut member that is operatively associated with the piston and rod assembly and moveable along a length of the threaded shaft when the threaded shaft is rotated. (e) A trigger for releasably retaining the piston and rod assembly in the rearward position. Activating the trigger causes the piston and rod assembly to be released for movement by the gas charge to the forward position so as to impact the plunger directly or indirectly with a force sufficient to cause the plunger to move in the forward direction to expel the medicament through the outlet. The gas tight chamber is adapted to minimize or prevent the escape of the gas charge so as to maintain the, gas charge in a pressurized state.
