A gas piston operating system for an autoloading firearm. The gas piston system may include a barrel having a longitudinally-extending bullet pathway, a gas block defining a piston bore, a passageway fluidly connecting the bore with the bullet pathway for diverting combustion gas from the pathway to the bore upon discharging the firearm, and a piston slidably disposed in the bore for reciprocating movement. In one embodiment, the piston includes a head having an axially-extending protrusion projecting towards the passageway. The protrusion is configured for slidable insertion into the passageway. The piston is movable from a first actuation position in which the protrusion is inserted into the passageway to a second actuation position in which the protrusion is at least partially withdrawn from the passageway. The protrusion acts to pretension a mechanical linkage between the piston and a reciprocating bolt assembly.