Jet injector having air pocket in the tip

A method for injecting a fluid medicament (24) into a patient uses a syringe having a plunger that can be advanced into the syringe chamber (12). As the plunger is advanced, the medicament is expelled from the chamber through an injection tube that extends from the chamber. Preferably, the injection tube is formed as a cone-shaped funnel with a taper of generally decreasing radius in a distal direction. The steps for the method include creating a gas pocket (22) in the distal portion of the injection tube. Then, as the plunger is advanced into the chamber to accelerate the fluid medicament through the injection tube, the gas pocket allows the accelerating fluid medicament to gain momentum. The change in this momentum as the fluid medicament collides with the skin will generate an impulse force that is manifested as an initial pressure spike. This pressure spike creates a hole in the skin of the patient as the fluid medicament exits the tube. Further advancement of the plunger into the chamber maintains a substantially constant infusion pressure for injecting the fluid medicament through the hole and into the patient.