A needleless injection system and method for injecting a medical product comprises a needleless injection device having a nozzle assembly defining an ampule chamber for holding medication, an energy device for pushing medication out of the nozzle assembly, and an energy device actuating mechanism. These components are operatively contained in a housing configured for a hand held operation. The energy device is a self-contained gas spring operatively connected to a plunger operating within the ampule chamber. The gas spring is preferably stepped which can be used in many different applications, an air gun being one. The injection device can be provide with a coupling device for coupling a medication supply vial to the nozzle assembly. The nozzle assembly is removably situated in the housing with the plunger removable therewith.