A piston for a gas-operated firearm has an elongated body having a head portion adapted to be closely received in the cylindrical bore for reciprocation within the bore, the head portion having a first side toward a forward direction and a second side toward the rearward direction, the body having an operational facility on the second side adapted to transmit an operating force to the action in response to gas pressure on the first side, and the head portion defining a bypass passage between the first side and the second side. The bypass passage may have an adjustment facility adapted to provide selectable resistance to gas flow through the bypass passage. The adjustment facility may be a movable occlusion element that selectably occludes the bypass passage. The occlusion element may be a threaded element. The occlusion element may be a collar that receives a portion of the body.