Plunger sub-assemblies and auto-injectors having low retraction activation force

An automatic injector comprising a housing, an activation mechanism, an actuation mechanism, and a syringe cartridge having a plunger sub-assembly and a needle assembly, wherein the housing includes a recess on an inner surface of the housing, the actuation mechanism comprises an actuation biasing member residing in an initial energized state substantially within an upper portion of an actuation pill configured to apply an injection force to the plunger sub-assembly to expel liquid from a drug chamber of the syringe cartridge, and the plunger sub-assembly comprises a plunger outer having at least one resiliently flexible engagement prong, a plunger inner having a shoulder, and a plunger biasing member retained in a first energized state between said plunger outer and plunger inner, wherein the at least one engagement prong of the plunger outer is configured to interface with a recess on the inner surface of the housing and flex radially outward to disengage from the shoulder of the plunger inner without an additional force applied by the actuation mechanism on the plunger subassembly, and wherein disengagement of the at least one engagement prong from the shoulder of the plunger inner allows the plunger biasing member to expand from the first energized state to a second expanded state to retract the plunger inner with respect to the plunger outer.