Autoinjectors piston subassemblies and having low activation force retraction

A subset of piston actuating force of low shrinkage (200) for an automatic injector (100) including: an outer plunger (220) having one or more engagement teeth (220A), an inner piston (210) has a projection (210A), and a pusher piston (270) retained in a first energized state between said outer piston (220) and inner piston (210) when engagement teeth (220A) of the outer piston (220) engage detachably with the projection (210A) of the inner piston (210), where the disengagement of the engagement teeth (220A) with the projection (210A) of the inner piston (210) allows the pushing member piston (270) expands from the first energized state to a second expanded state to remove the inner piston (210) relative to the outer piston (220).