A fluid injection device, comprising: a housing comprising an inner surface and an outer surface which extend from a proximal trigger end to a distal dispensing end along an injection axis; a housing top retainer at said trigger end with a lower stop socket, an upper pop-in socket, a venting slot, an external actuation surface disposed transverse to the injection axis at the trigger end, the external actuation surface comprising a surface that tapers downward toward the dispensing end, and a safety cap with a pivot hinge and a snap-in recess lock; wherein said venting slot is positioned at a distal portion of said housing top retainer near said lower stop socket; a syringe adapter at the dispensing end of the housing, the syringe adapter configured to retain a needle-free syringe such that a plunger of the needle-free syringe is movable within the housing along the injection axis; wherein said syringe adapter comprises an outer surface that extends away from the injection axis beyond the outer surface of the housing; a piston retained within the housing by the syringe adapter and movable along the injection axis between a loaded position near the trigger end and an unloaded position near the dispensing end, the piston comprising a piston head near the dispensing end and a trigger catch near the trigger end of the housing, the piston head being movably positioned to bias the plunger of the needle-free syringe via the piston head along the injection axis; a compression spring associated with the piston, the piston and compression spring sized relative to the housing such that the compression spring is compressible along the injection axis within the housing; and a trigger mounted within the housing near the trigger end and movable between a non-actuated position and an actuated position by depression of a push-button disposed at the external actuation surface of the housing top retainer,the trigger comprising a trigger latch configured to engage the trigger catch when the trigger is in the non-actuated position and the piston is in the loaded position and to release the trigger catch when the trigger is in the actuated position, wherein the trigger catch is configured to urge the trigger latch in a direction transverse to the injection axis when the piston moves from the unloaded position to the loaded position. a housing comprising an inner surface and an outer surface which extend from a proximal trigger end to a distal dispensing end along an injection axis; a housing top retainer at said trigger end with a lower stop socket, an upper pop-in socket, a venting slot, an external actuation surface disposed transverse to the injection axis at the trigger end, the external actuation surface comprising a surface that tapers downward toward the dispensing end, and a safety cap with a pivot hinge and a snap-in recess lock; wherein said venting slot is positioned at a distal portion of said housing top retainer near said lower stop socket; a syringe adapter at the dispensing end of the housing, the syringe adapter configured to retain a needle-free syringe such that a plunger of the needle-free syringe is movable within the housing along the injection axis; wherein said syringe adapter comprises an outer surface that extends away from the injection axis beyond the outer surface of the housing; a piston retained within the housing by the syringe adapter and movable along the injection axis between a loaded position near the trigger end and an unloaded position near the dispensing end, the piston comprising a piston head near the dispensing end and a trigger catch near the trigger end of the housing, the piston head being movably positioned to bias the plunger of the needle-free syringe via the piston head along the injection axis; a compression spring associated with the piston, the piston and compression spring sized relative to the housing such that the compression spring is compressible along the injection axis within the housing; and a trigger mounted within the housing near the trigger end and movable between a non-actuated position and an actuated position by depression of a push-button disposed at the external actuation surface of the housing top retainer,the trigger comprising a trigger latch configured to engage the trigger catch when the trigger is in the non-actuated position and the piston is in the loaded position and to release the trigger catch when the trigger is in the actuated position, wherein the trigger catch is configured to urge the trigger latch in a direction transverse to the injection axis when the piston moves from the unloaded position to the loaded position.