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, of the type comprising a plunger slidably received in said dosing reservoir and movable in a forward direction for expelling said medicament through said orifice, said injection device comprising an actuating device configured to control the movement of the plunger, said actuating device comprising: (a) a gas tight chamber; (b) a piston and rod assembly slidably received in said gas tight chamber and movable between a forward position and a rearward position, wherein the rod extends through an opening in said gas tight chamber to connect directly to the plunger via a gap-coupling or indirectly to the plunger via a gap-coupling and a transferring means; (c) a gas charge in said gas tight chamber for urging said piston and rod assembly to said forward position; (d) a charging mechanism configured to move said piston and rod assembly against said gas charge into a charged position, said charged position being between said forward position and said rearward position set according to a desired charge, said charging mechanism comprising a threaded shaft threadingly engaged with nut member that is directly or indirectly connectable to the gap-coupling and is moveable along a length of the threaded shaft between a forward position and a rearward position when said threaded shaft is rotated, wherein charging is achieved by movement of the nut member from said forward position towards said rearward position, which in turn moves the gap-coupling and the piston and rod assembly from its forward position to its charged position; and (e) a trigger for releasably retaining said piston and rod assembly in said charged position, wherein the nut member is moveable to its forward position when said piston and rod assembly is retained in its charged position, wherein said trigger comprises a trigger plate including a locking passage through which a portion of the rod extends, said trigger plate being movable between a locked position and an unlocked position, wherein in the unlocked position said locking passage is generally axially aligned with said rod to permit axial movement of the rod, and wherein in said locked position said trigger plate is generally out of axial alignment with said rod such that an inner surface of said locking passage is frictionally engaged with a surface of said rod thereby retaining said piston and rod assembly in said charged position; whereby activating said trigger causes said piston and rod assembly to be released for movement by said gas charge to said forward position so as to impact said plunger directly or indirectly via the transferring means with a force sufficient to cause said plunger to move in said forward direction to expel said medicament through said outlet and whereby said gas tight chamber is adapted to minimize or prevent the escape of said gas charge so as to maintain said gas charge is a pressurized state. (a) a gas tight chamber; (b) a piston and rod assembly slidably received in said gas tight chamber and movable between a forward position and a rearward position, wherein the rod extends through an opening in said gas tight chamber to connect directly to the plunger via a gap-coupling or indirectly to the plunger via a gap-coupling and a transferring means; (c) a gas charge in said gas tight chamber for urging said piston and rod assembly to said forward position; (d) a charging mechanism configured to move said piston and rod assembly against said gas charge into a charged position, said charged position being between said forward position and said rearward position set according to a desired charge, said charging mechanism comprising a threaded shaft threadingly engaged with nut member that is directly or indirectly connectable to the gap-coupling and is moveable along a length of the threaded shaft between a forward position and a rearward position when said threaded shaft is rotated, wherein charging is achieved by movement of the nut member from said forward position towards said rearward position, which in turn moves the gap-coupling and the piston and rod assembly from its forward position to its charged position; and (e) a trigger for releasably retaining said piston and rod assembly in said charged position, wherein the nut member is moveable to its forward position when said piston and rod assembly is retained in its charged position, wherein said trigger comprises a trigger plate including a locking passage through which a portion of the rod extends, said trigger plate being movable between a locked position and an unlocked position, wherein in the unlocked position said locking passage is generally axially aligned with said rod to permit axial movement of the rod, and wherein in said locked position said trigger plate is generally out of axial alignment with said rod such that an inner surface of said locking passage is frictionally engaged with a surface of said rod thereby retaining said piston and rod assembly in said charged position; whereby activating said trigger causes said piston and rod assembly to be released for movement by said gas charge to said forward position so as to impact said plunger directly or indirectly via the transferring means with a force sufficient to cause said plunger to move in said forward direction to expel said medicament through said outlet and whereby said gas tight chamber is adapted to minimize or prevent the escape of said gas charge so as to maintain said gas charge is a pressurized state.