Abstract: An apparatus for delivering an injectate to a patient includes a piston mechanism including an outer piston. The outer piston is slideably disposed in a chamber of the apparatus and has a channel extending therethrough. The piston mechanism also includes an inner piston including a stop and a plunger element, the plunger element being slideably disposed in the channel of the outer piston. The piston mechanism is configurable into a number of configurations including a first configuration in which the stop of the inner piston is not in contact with the outer piston such that the inner piston is movable in the channel without causing movement of the outer piston and a second configuration in which the stop of the inner piston is in contact with the outer piston such that the inner piston and the outer piston move together in the chamber.