 Moreover, the gas piston operation of the AR-18 proved much more resistant to carbon fouling than the direct gas impingement system of the earlier AR-10 and AR-15 rifles, as it does not vent gas and carbon particles directly into the receiver.The AR-18's action is powered by a short-stroke gas piston above the barrel.The gas piston is of 3-piece design to facilitate disassembly, with a hollow forward section with 4 radial gas vent holes fitting around a stainless steel gas block projecting rearwards from the foresight housing.