Direct impingement is a type of gas operation for a firearm that directs gas from a fired cartridge indirectly (through the barrel, through a gas block, and then through a gas tube) into the bolt carrier or slide assembly to cycle the action.
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Firearms featuring a direct impingement design are, in principle, capable of being constructed so as to be slightly lighter than their piston operated counterparts (due to the lack of any additional hardware other than the gas tube, which is required to channel the gas from the barrel back towards the action). One undesirable consequence of this design is that the core components of the firearm are subject to a greater degree of fouling (as compared to a firearm employing a long or short stroke piston) due to the direct exposure of the impinging region of the bolt carrier to the propellant gases each time the firearm cycles. As a consequence of this, such firearms are ostensibly subject to less reliable operation (in the sense that all malfunctions save for those caused by fouling in the action are disregarded) when compared with piston operated designs, though in practice many factors might influence how significantly a direct impingement design might accelerate fouling in the chamber, and how that fouling might alter the reliability of the firearm. The usual example one might find when searching for a direct impingement gas system is the one used in the AR-15 style rifle, first patented by ArmaLite for use in the AR-10. This is likely not the best example though, as the subtleties involved in ArmaLite's patent on the gas system significantly diverge from classical direct impingement; upon firing, the pressurized propellant gasses exit the barrel via the gas port and travel the length of the gas tube, but instead of simply applying the inertia necessary to cycle the weapon directly to the bolt carrier, the gas is funneled inside the bolt carrier wherein the increase in pressure results in the bolt itself acting as a piston, forcing the bolt carrier away from the barrel face (the purpose of the gas rings on an AR-10/AR-15 style bolt is to ensure a proper seal is achieved between the bolt and the bolt carrier).
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 manufacturing costs, and reducing the mass of the operating parts, and thereby the wear on mechanical parts due to movement. By removing the gas piston, the potential amount of moving mass is lowered, thus decreasing the potential for a disrupted sight picture.
The main disadvantage of direct impingement is that the breech of the firearm's firing mechanism becomes fouled more quickly due to being exposed to the residues of the burned cartridge propellant. This is caused by particles suspended in 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 the relatively cooler operating parts. These deposits increase friction on the bolt's camming system leading to malfunctions, so that frequent and thorough 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 causes essential lubricant to be "burned off". Lack of proper lubrication is the most common source of weapon malfunctions. These combined factors reduce service life of these parts, reliability, and mean time between failures.
The operation of the system is highly dependent on the length of both barrel and gas tube which transports gas from the barrel to the bolt. Using too short a gas tube can result in increased pressure inside the bolt assembly and increased rate of automatic fire, both of which can have detrimental effect on the weapon and accuracy of shots. The use of a suppressor also increases gas pressure, further aggravating the situation. The problem can be reduced by using a longer gas tube, moving the gas port on the barrel further forward, and/or by installing an adjustable gas block to provide the right amount of gas pressure depending on the desired operating mode.
The first experimental rifle using a direct impingement system was the French Rossignol ENT B1 automatic 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 Automatgevär 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.
The original AR-10 action (later developed into the ArmaLite AR-15 and M16 rifle) designed by Eugene Stoner is commonly called a direct impingement system, but it does not utilize a conventional direct impingement system. In U.S. Patent 2,951,424 , the designer statesThis invention is a true expanding gas system instead of the conventional impinging gas system.? Gas is routed from a port in the barrel through a gas tube, directly to a chamber inside the bolt carrier. The bolt within the bolt carrier is fitted with piston rings to contain the gas. In effect, the bolt and carrier act as a gas piston and cylinder.