Abstract: An improved gas operated firearm utilizing a metering device positioned for regulating the flow of gas from the barrel of the firearm to a piston which operates the action of the firearm. The metering device consists of a metering plug which has an annular rib where the plug is inserted into a cylinder which communicates with the piston which operates the action of the firearm. The area of the gas flow opening between the rib of the metering plug and the bore of the cylinder is thus selectable to one of the flow of gases. The metering plug may also have a plurality of flats on its surface of different dimensions and the metering plug is adjustable to selected positions for varying the degree of restriction posed to gases passing beteween the plug and the bore of the cylinder.