Direct impingement is a type of

gas operation Gas-operation is a system of operation used to provide energy to operate locked breech, autoloading firearms. In gas-operation, a portion of high-pressure gas from the cartridge being fired is used to power a mechanism to dispose of the spent ...

for a

firearm A firearm is any type of gun designed to be readily carried and used by an individual. The term is legally defined further in different countries (see Legal definitions). The first firearms originated in 10th-century China, when bamboo tubes ...

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. Firearms using direct impingement are theoretically lighter, more accurate, and less expensive than firearms using cleaner and cooler gas piston systems.

Advantages

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). 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 firearm movement and barrel distortion before the bullet leaves the barrel. In the AR-10 and AR-15 designs, the gas directed out of the bolt carrier group may improve reliability in some conditions. The jet of gas can blow debris away from the ejection port, preventing ingress of material which, in other designs, would infiltrate the mechanism and foul the gun.

Disadvantages

The main disadvantage of direct impingement is that the

breech Breech may refer to: * Breech (firearms), the opening at the rear of a gun barrel where the cartridge is inserted in a breech-loading weapon * breech, the lower part of a pulley block * breech, the penetration of a boiler where exhaust gases leave ...

of the firearm's firing mechanism becomes fouled more quickly than

long Long may refer to: Measurement * Long, characteristic of something of great duration * Long, characteristic of something of great length * Longitude (abbreviation: long.), a geographic coordinate * Longa (music), note value in early music mensu ...

or short stroke piston firearms by direct exposure of the impinging region of the bolt carrier to the residues of the burned cartridge propellant each time the firearm cycles. 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. 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 cause of weapon malfunctions. These combined factors reduce service life of these parts, reliability, and

mean time between failures Mean time between failures (MTBF) is the predicted elapsed time between inherent failures of a mechanical or electronic system during normal system operation. MTBF can be calculated as the arithmetic mean (average) time between failures of a system ...

Variables

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 A silencer, also known as a sound suppressor, suppressor, or sound moderator, is a muzzle device that reduces the acoustic intensity of the muzzle report (sound of a gunshot) and muzzle rise when a gun (firearm or air gun) is discharged, b ...

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.

History

The first experimental rifle using a direct impingement system was the French

Rossignol ENT The Rossignol ENT was an experimental automatic rifle of French origin from the first years of the 20th century. It was the first rifle which used the direct impingement operating system, which found later use in the Swedish Ag m/42, the French ...

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.

Stoner bolt and carrier piston system

A widely known direct impingement gas system is the one used in the

AR-15 style rifle An AR-15-style rifle is any lightweight semi-automatic rifle based on the Colt AR-15 design. The original ArmaLite AR-15 is a scaled-down derivative of Eugene Stoner's ArmaLite AR-10 design. The then Fairchild Engine and Airplane Corporation ...

, first patented by

ArmaLite ArmaLite, or Armalite, is an American small arms engineering company, formed in the early 1950s, in Hollywood, California. Many of its products, as conceived by chief designer Eugene Stoner, relied on unique foam-filled fiberglass butt/stock fu ...

for use in the AR-10. The original AR-10 action (later developed into the

ArmaLite AR-15 The ArmaLite AR-15 is a select-fire, gas-operated, air-cooled, magazine-fed rifle manufactured in the United States between 1959 and 1964. Designed by American gun manufacturer ArmaLite in 1956, it was based on its AR-10 rifle. The ArmaLit ...

M16 rifle The M16 rifle (officially designated Rifle, Caliber 5.56 mm, M16) is a family of military rifles adapted from the ArmaLite AR-15 rifle for the United States military. The original M16 rifle was a 5.56×45mm automatic rifle with a 20-r ...

and

M4 carbine The M4 carbine (officially Carbine, Caliber 5.56 mm, M4) is a 5.56×45mm NATO, gas-operated, magazine-fed carbine developed in the United States during the 1980s. It is a shortened version of the M16A2 assault rifle. The M4 is extensive ...

) designed by

Eugene Stoner Eugene Morrison Stoner (November 22, 1922 – April 24, 1997) was an American firearms designer who is most associated with the development of the ArmaLite AR-15 rifle that was redesigned and modified by Colt's Patent Firearm Company (now known ...

is commonly called a direct impingement system, but it does not utilize a conventional direct impingement system. In , the designer states: ″This 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 ring A piston ring is a metallic split ring that is attached to the outer diameter of a piston in an internal combustion engine or steam engine. The main functions of piston rings in engines are: # Sealing the combustion chamber so that there is min ...

s to contain the gas. In effect, the bolt and carrier act as a gas piston and cylinder. The subtleties involved in ArmaLite's patent on the gas system Eugene Stoner, 1956, Gas operated bolt and carrier system, US2951424A, https://patents.google.com/patent/US2951424A/en 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.

References

Sources

* Centre des archives de l'armement, Châtellerault. National Armament Archives Center. * Huon, Jean. Proud Promise—French Semiautomatic Rifles: 1898-1979, Collector Grade Publications,1995,{{ISBN, 0-88935-186-4 * United States Patent Office, Patent No. 2951424 - Gas Operated Bolt and Carrier System, Sep 6 1960.

External links

How Does It Work: Direct Gas Impingement
Forgotten Weapons Firearm actions