Direct impingement is a type of gas operation for a firearm that utilizes gas from a fired cartridge to impart force on 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.
Firearms with a direct impingement design can, in principle, be constructed lighter than piston-operated designs. Because high-pressure gas acts directly upon the bolt and carrier in a direct impingement system, it does not need a separate gas cylinder, piston, and operating rod assembly of a conventional piston-operated system, only requiring a gas tube to channel gas from the barrel back towards the action. This saves weight, lowers manufacturing costs, and reduces 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 also lowered, decreasing the potential for firearm movement and barrel distortion before the bullet leaves the barrel. [1]
The gas directed at the bolt carrier group may improve reliability in some conditions. The jet of gas can blow debris away from the ejection port which, in other designs, could enter the mechanism and foul the gun.
The main disadvantage of direct impingement is that the breech of the firing mechanism becomes fouled more quickly compared to long or short stroke piston systems. This is caused by the operating mechanism's direct exposure to gases from burned cartridge propellant when the firearm cycles. The gases contain vaporized metals, carbon, and impurities in a gaseous state until they condense on the relatively cooler operating parts. These deposits increase friction on the bolt's camming system, leading to malfunctions. 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. [2]
Another disadvantage of direct impingement is that combustion gases heat the bolt and bolt carrier as the firearm operates. This heating causes 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. [3]
The operation of the system is highly dependent on both the length of the barrel and the 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 an increased rate of automatic fire, both of which can negatively affect 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. [1]
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 French MAS 40 rifle adopted in March 1940. The Swedish Automatgevär m/42 is another 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 designed by Eugene Stoner (later developed into the ArmaLite AR-15, M16 rifle, and M4 carbine) is commonly called a direct impingement system, but it does not actually utilize this mechanism. In U.S. patent 2,951,424 , the designer states: ″This invention is a true expanding gas system instead of the conventional impinging gas system.″ [4] 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. The subtleties involved in ArmaLite's patent on the gas system [5] 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. [6] The Stoner bolt and carrier piston system is ammunition specific since it does not have an adjustable gas port or valve to adjust the weapon to various propellant and projectile or barrel length specific pressure behavior.
In firearms terminology, an action is the functional mechanism of a breech-loading firearm that handles the ammunition cartridges, or the method by which that mechanism works. Actions are technically not present on muzzleloaders, as all those are single-shot firearms with a closed off breech with the powder and projectile manually loaded from the muzzle. Instead, the muzzleloader ignition mechanism is referred to as the lock.
The Colt AR-15 is a lightweight, magazine-fed, gas-operated semi-automatic rifle. It is a semi-automatic version of the M16 rifle sold for the civilian and law enforcement markets in the United States. The AR in AR-15 stands for ArmaLite rifle, after the company that developed it in the 1950s. Colt's Manufacturing Company currently owns the AR-15 trademark, which is used exclusively for its line of semi-automatic AR-15 rifles.
The La France M16K is an M16 rifle modified by the company La France Specialties, which among with other firearm-related activities, convert common military weapons into more compact configurations typically for law enforcement and special forces use.
The ArmaLite AR-10 is a 7.62×51mm NATO battle rifle designed by Eugene Stoner in the late 1950s and manufactured by ArmaLite. When first introduced in 1956, the AR-10 used an innovative combination of a straight-line barrel/stock design with phenolic composite, a new patent-filed gas-operated bolt and carrier system and forged alloy parts resulting in a small arm significantly easier to control in automatic fire and over 1 lb (0.45 kg) lighter than other infantry rifles of the day. Over its production life, the original AR-10 was built in relatively small numbers, with fewer than 10,000 rifles assembled. However, the ArmaLite AR-10 would become the progenitor for a wide range of firearms.
The MAS-49 is a French semi-automatic rifle that replaced various bolt-action rifles as the French service rifle that was produced from 1949. It was designed and manufactured by the government-owned MAS arms factory. The French Army formal designation of the MAS-49 is Fusil semi-automatique 7 mm 5 M. 49.
The Stoner 63 is a 5.56×45mm NATO modular weapon system. Using a variety of modular components, it can be configured as an assault rifle, carbine, top-fed light machine gun, belt-fed squad automatic weapon, or as a vehicle mounted weapon. Also known as the M63, XM22, XM23, XM207 or the Mk 23 Mod 0 machine gun, it was designed by Eugene Stoner in the early 1960s. Cadillac Gage was the primary manufacturer of the Stoner 63 during its history. The Stoner 63 saw very limited combat use by United States forces during the Vietnam War. A few were also sold to law enforcement agencies.
Eugene Morrison Stoner was an American machinist and 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 for the United States military as the M16 rifle.
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The Z-M LR-300 is an American rifle designed by gunsmith Allan Zitta and manufactured by Z-M Weapons. The model name LR-300 stands for Light Rifle and 300 is for 300 meters, which is regarded by the manufacturer as the effective range of the rifle with a standard 55 gr (3.6 g) FMJ bullet. The design is based on the AR-15, M16 and C7 rifles, but has a unique semi-direct gas impingement system and a folding stock option.
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 case and insert a new cartridge into the chamber. Energy from the gas is harnessed through either a port in the barrel or a trap at the muzzle. This high-pressure gas impinges on a surface such as a piston head to provide motion for unlocking of the action, extraction of the spent case, ejection, cocking of the hammer or striker, chambering of a fresh cartridge, and locking of the action.
A bolt is the part of a repeating, breechloading firearm that blocks the rear opening (breech) of the barrel chamber while the propellant burns, and moves back and forward to facilitate loading/unloading of cartridges from the magazine. The firing pin and extractor are often integral parts of the bolt. The terms "breechblock" and "bolt" are often used interchangeably or without a clear distinction, though usually, a bolt is a type of breechblock that has a nominally circular cross-section.
The Barrett REC7 is an American firearm manufactured as a selective-fire fully-automatic and semi-automatic rifle by Barrett Firearms. It is an M4 carbine utilizing a short-stroke gas piston system. REC7 is available in either 5.56×45mm NATO or 6.8mm Remington SPC. The REC7 is Barrett's second AR-pattern rifle chambered for the 6.8mm Remington SPC cartridge, the first being the Barrett M468 rifle. The 6.8 SPC-chambered M468 rifle employed the same direct gas-impingement system as the M4.
The T65 is a rifle developed and manufactured by the Combined Logistics Command of the Republic of China Armed Forces in Taiwan. Originally patterned after the Armalite AR-18 that has a short-stroke gas system, the prototype unveiled in 1975 showed a rifle that is heavily influenced by the AR-15 family of rifles, albeit with modified iron sights, a reshaped stock, and redesigned handguards. The designated number '65' refers to the Year 65 of Republic of China (1976), the year the rifle's design was finalized.
A muzzle booster or recoil booster is a device fixed to the muzzle of a firearm, intended to harness the energy of the escaping propellant to augment the force of recoil on portions of the firearm. In spite of its name, a muzzle booster does not increase muzzle force or velocity but instead is usually used to improve the reliability and/or rate of fire of a recoil operated firearm. It was invented by Hiram Maxim in 1894.
The Mendoza RM2 was a light machine gun similar to the M1918 BAR manufactured in Mexico by Productos Mendoza, S.A. Rafael Mendoza have been producing machine guns for the Mexican Army since 1933 and all have been noted for their lightness, simplicity, ease of maintenance, and economic construction without sacrificing reliability.
The SG 540 is a 5.56×45mm NATO assault rifle developed in the early 1970s by Schweizerische Industrie Gesellschaft of Neuhausen, Switzerland as a private venture primarily destined for export markets and as a potential replacement for the 7.5×55mm Swiss SG 510 automatic rifle known as the Stgw 57 in Swiss service.
The following are terms related to firearms and ammunition topics.
Recoil operation is an operating mechanism used to implement locked-breech autoloading firearms. Recoil operated firearms use the energy of recoil to cycle the action, as opposed to gas operation or blowback operation using the pressure of the propellant gas.
UDMC PVAR rifles are Filipino variants of the Armalite AR-15 and AR-10, using the Pneumatic Valve and Rod system manufactured by United Defense Manufacturing Corporation.
Patriot Ordnance Factory (POF-USA) manufactures and distributes law enforcement, military and civilian rifles in Phoenix, Arizona, and was the first manufacturer and seller of gas-piston-operated weapon systems on AR-style rifles in the United States.