Direct impingement is a type of gas operation for a firearm

Firearm

A firearm is a weapon that launches one, or many, projectile at high velocity through confined burning of a propellant. This subsonic burning process is technically known as deflagration, as opposed to supersonic combustion known as a detonation. In older firearms, the propellant was typically...

 that directs gas from a fired cartridge

Cartridge (firearms)

A cartridge, also called a round, packages the bullet, gunpowder and primer into a single metallic case precisely made to fit the firing chamber of a firearm. The primer is a small charge of impact-sensitive chemical that may be located at the center of the case head or at its rim . Electrically...

 directly to the bolt

Bolt (firearm)

A bolt is a mechanical part of a firearm that blocks the rear of the chamber while the propellant burns.In manually-operated firearms, such as bolt-action, lever-action, and pump-action rifles and shotguns, the bolt is held fixed by its locking lugs during firing, forcing all the expanding gas...

 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

Breech-loading weapon

A breech-loading weapon is a firearm in which the cartridge or shell is inserted or loaded into a chamber integral to the rear portion of a barrel....

 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). While the 7.62mm Armalite

ArmaLite

ArmaLite is the name of a small arms engineering facility founded in the early 1950s, and once associated with the Fairchild Engine and Airplane Corporation. ArmaLite was formally incorporated as a subdivision of Fairchild on October 1, 1954...

 AR-10

AR-10

The AR-10 is an American 7.62 mm battle rifle developed by Eugene Stoner in the late 1950s at ArmaLite, then a division of the Fairchild Aircraft Corporation...

 proved fairly resistant to malfunctions

Firearm malfunction

A firearm malfunction is the partial or complete failure of a firearm to operate as intended. Malfunctions range from temporary and relatively safe situations, such as a casing that didn't eject, to potentially dangerous occurrences that may permanently damage the gun and cause injury or death...

 caused by deposit fouling, the smaller 5.56mm M16A1

M16 rifle

The M16 is the United States military designation for the AR-15 rifle adapted for both semi-automatic and full-automatic fire. Colt purchased the rights to the AR-15 from ArmaLite, and currently uses that designation only for semi-automatic versions of the rifle. The M16 fires the 5.56×45mm NATO...

 experienced significantly increased malfunctions due to fouling and carbon deposits after a change in propellant specifications.

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

Tempering

Tempering is a heat treatment technique for metals, alloys and glass. In steels, tempering is done to "toughen" the metal by transforming brittle martensite or bainite into a combination of ferrite and cementite or sometimes Tempered martensite...

 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. Heat can also melt the lacquer coatings of steel cartridge cases, gumming up parts. Thermal expansion

Thermal expansion

Thermal expansion is the tendency of matter to change in volume in response to a change in temperature.When a substance is heated, its particles begin moving more and thus usually maintain a greater average separation. Materials which contract with increasing temperature are rare; this effect is...

 in the action can result in loss of tolerances and consequent degradation in accuracy.

Conversely, in a conventional gas piston design such as the Armalite AR-18, 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

M1 Carbine

The M1 carbine is a lightweight, easy to use semi-automatic carbine that became a standard firearm for the U.S. military during World War II, the Korean War and the Vietnam War, and was produced in several variants. It was widely used by U.S...

 and FN SCAR

FN SCAR

The Special Operations Forces Combat Assault Rifle, or SCAR, is a modular rifle made by FN Herstal for the United States Special Operations Command to satisfy the requirements of the SCAR competition. This family of rifles consist of two main types...

.

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

Ag m/42

The Automatgevär m/42 is a Swedish semi-automatic rifle in limited use by the Swedish Army from 1942 until the 1960s.-History:...

 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 premier current example of direct impingement is the AR-15/M16 rifle designed by Eugene Stoner

Eugene Stoner

Eugene Morrison Stoner is the man most associated with the design of the AR-15, which was adopted by the US military as the M16...

who first designed the Armalite AR-10. In the Stoner system covered by , 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.