Direct impingement Direct impingement is a type of gas operation for a firearm that directs gas from a fired cartridge directly to the bolt carrier or slide assembly to cycle the action . Evaluation . Unlike conventional gas - operated firearms , direct impingement does away with a separate gas cylinder , piston , and operating rod assembly . High - pressure gas acts directly upon the bolt and carrier thereby saving weight , lowering costs , and reducing the mass of the operating parts . The main disadvantage of direct impingement is that the breech of the firearm becomes fouled more quickly . This is caused by solids from the high - temperature gas condensing on the bolt face and primary operating mechanism . The combustion gases contain vaporized metals , carbon , and impurities in a gaseous state until they contact cooler operating parts . The deposits increase friction on the bolt ' s camming system leading to jams , so that thorough and frequent cleaning is required to ensure reliability . The amount of fouling depends upon the rifle ' s design as well as the type of propellant powder used . For example , the French MAS 44 and MAS 49 series of rifles was known to have been successfully operated for years with corrosive - primed ammunition using ordinary field cleaning expedients such as gasoline ( as solvent ) and straight - grade motor oil ( as lubricant ) . . A further disadvantage of direct impingement is that combustion gases heat the bolt and bolt carrier as the firearm operates . This heating may alter the temper of metal parts , accelerating wear and decreasing the service life of the bolt , extractor , and extractor spring . Heat dries up lubricant and makes the operating parts difficult to handle when clearing malfunctions . Thermal expansion in the action can result in loss of tolerances and consequent degradation in accuracy . Conversely , in a conventional gas piston design such as the H & K 416 , gas used to operate the firearm is isolated from the breech and contained within the gas cylinder then vented away from other working parts . Some systems actually contain the gas completely and vent excess back into the barrel , such as the M1 carbine and FN SCAR . * History . The first experimental rifle using a direct impingement system was the French ENT 1901 Rossignol B1 rifle followed by Rossignol ' s B2 , B4 and B5 . The first successful production weapon was the MAS 40 rifle adopted in March 1940 . The Swedish Ag m / 42 is another well - known example . Both the French and Swedish rifles use a simple system whereby the gas tube acts as a piston with a cylinder recess in the bolt carrier . It is incorrectly believed that the AR - 15 / M16 rifle designed by Eugene Stoner uses a direct impingement action . In the Stoner system covered by Stoner specifically states that the action is not direct impingement saying ″ This invention is a true expanding gas system instead of the conventional impinging gas system . ″ Gas is routed from a port in the barrel directly to a chamber formed in the bolt carrier . The bolt acts as the piston and is sealed with small automobile - style piston rings . It is a gas piston system without an operating rod .