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Gas-operated firearm (long-stroke piston, e.g. AK-47). 1) gas port, 2) piston head, 3) rod, 4) bolt, 5) bolt carrier, 6) spring

Gas-operation system of operation used to provide energy to operate autoloading firearms. In gas-operation, a portion of high pressure gas from the cartridge being fired is used to power a mechanism to extract the spent case and chamber a new cartridge. Energy from the gas is harnessed through either a port in the barrel or 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.

Gas systems

Most current gas systems employ some type of piston. The face of the piston is acted upon by gas from the combustion of the propellant from the barrel of the firearm. Early methods such as Browning's 'flapper' prototype, the Bang rifle, and Garand rifle used relatively low-pressure gas from at or near the muzzle. This, combined with more massive operating parts, reduced the strain on the mechanism. To simplify and lighten the firearm, gas from nearer the chamber needed to be used. This gas is of extremely high pressure and has sufficient force to destroy a firearm unless it is regulated somehow. Several methods are employed to regulate the energy. The M1 carbine incorporates a very short piston, or "tappet". This movement is closely restricted by a shoulder recess. Excess gas is then vented back into the bore. The M14 rifle and M60 GPMG use the White expansion and cutoff system to stop (cut off) gas from entering the cylinder once the piston has traveled a short distance.[1] Most systems, however, vent excess gas into the atmosphere through slots, holes, or ports.

Long-stroke piston

Diagram of long-stroke gas operation system

Long stroke gas piston, from an AK-74 assault rifle.

With a long-stroke system, the piston is mechanically fixed to the bolt group and moves through the entire operating cycle. This system is used in weapons such as the Bren light machine gun, AK-47, M249 Squad Automatic Weapon, FN MAG and M1 Garand. The primary advantage of the long-stroke system, beyond design simplicity and robustness, is that the mass of the piston rod adds to the momentum of the bolt carrier enabling more positive extraction, ejection, chambering, and locking; Also as the gas is not directed back into the chamber the weapon stays cleaner longer thus reducing the likelihood of a malfunction. The primary disadvantage to this system is the disruption of the point of aim due to the center of mass changing during the action cycle and energetic and abrupt stops at the beginning and end of bolt carrier travel.

direct impingement

short-stroke gas piston

Direct impingement

The direct impingement method of operation vents gas through a tube to the working parts of a rifle where they directly impinge on the bolt carrier. Rifles that use this system include the Swedish Ljungman Ag m/42, U.S. M16 and French MAS-49. This system has the advantage of having the absolute minimum of recoiling action parts, resulting in the minimum possible weapon disturbance due to balance shifting during the action cycle as well as reducing overall weapon weight. It has the disadvantage of the propellant gas (and the accompanying fouling) being blown directly into the action parts.[2] The DI operation increases the amount of heat that is deposited in the receiver while firing of the M16 and causing essential lubricant to be "burned off".[3] This requires frequent and generous applications of appropriate lubricants.[4] Lack of proper lubrication is the most common source of weapon stoppages or jams.[5] A further disadvantage is that the bolt, extractor, ejector, pins, and springs are heated by this high-temperature gas. This heat may change the temper of the metal, resulting in reduced life of these parts and shortening time between failures.

Short-stroke piston

Short stroke gas piston and bolt carrier group, from a gas piston AR-15

With a short-stroke or tappet system, the piston moves separately from the bolt group. It may directly push[6] the bolt group parts as in the M1 carbine or operate through a connecting rod or assembly as in the Armalite AR-18 or the SKS. In either case, the energy is imparted in a short, violent push and the motion of the gas piston is then arrested allowing the bolt carrier assembly to continue through the operating cycle through kinetic energy. This has the advantage of reducing the total mass of recoiling parts compared to the long-stroke piston. This, in turn, enables better control of the weapon due to less mass needing to be stopped at either end of the bolt carrier travel.

Gas trap

A gas trap system is similar to long-stroke operation, however gas is 'trapped' after leaving the muzzle. The Bang rifle, early 'gas-trap Garand', and Gewehr 41 operated this way. These systems are longer, heavier, dirtier and more complex; however, they do use lower pressure gas and do not require that a hole be drilled in the barrel, two advantages that are largely negated by their disadvantages.

Gas assist systems

Floating chamber

Early machine guns were expensive to operate. The United States Army wanted to train machine gun crews with less-expensive ammunition. To do this, they needed the .22LR cartridge to operate firearms designed to use the .30-06 cartridge. David Marshall Williams invented a method that involved a separate floating chamber that acted as a gas piston with combustion gas impinging directly on the front of the floating chamber.[7] The Colt Service Ace conversion kit utilized this system, which allows a much heavier slide than other conversions operating on the unaugmented blowback mechanism. A floating chamber provides additional force to operate the heavier slide, providing a felt recoil level similar to that of a full power cartridge.[8]

Muzzle booster

Animation of the Vickers muzzle booster operation, showing the expanding gases pushing the barrel to the rear relative to the cooling jacket

The French Chauchat, German MG-42 machine gun, and some other recoil operated firearms use a gas trap style mechanism to provide additional energy to 'boost' the energy provided by recoil. This 'boost' provides higher rates of fire and/or more reliable operation. It is alternately called a gas assist, and may also be found in some types of blank-firing adapters.

Gas-delayed blowback

The bolt is not locked but is pushed rearward by the expanding propellant gases as in other blowback-based designs. However, propellant gases are vented from the barrel into a cylinder with a piston that delays the opening of the bolt. It is used by Volkssturmgewehr 1-5 rifle, the Heckler & Koch P7 and Steyr GB pistols.

Gas ejection

Patented by August Schuler, the Reform pistol featured a vertical row of barrels that advanced upwards with each shot exposing the fired chamber. As the lower barrel fired, a gas hole between the barrels pressurized the empty barrel enough to eject the case rearward. An extended spur on the hammer prevented the spent case from hitting the firer in the face. The final case required manual extraction.

Enhancements aimed at accuracy improvement in assault rifles

Some Soviet and later Russian designers have advocated a "balanced automatic" / "balanced assault rifle" principle (Russian: сбалансированной автоматики), where the gas extracted from the barrel is diverted in two directions: besides cycling the action, a counter-weight is pushed in the opposite direction, in order to minimize the recoil felt by the shooter. In the Soviet Union, the system was first proposed by VM Sabelnikov (В.М. Сабельников) and PA Tkachev (П.А. Ткачев) in the late 1960.[9] This system is used in the AEK-971 and the AK-107 for example.

The system used in AN-94 for the same accuracy-improvement purpose is more complex, involving a combination of gas operation and recoil operation.

Other autoloading systems

Other autoloading systems are:

  • Blow forward firearms lack the use of a bolt but instead a moving barrel that gets dragged forward by the bullet until it leaves the barrel to cycle its action.
  • Blowback firearms use the expanding gas impinging on the cartridge itself to push the bolt of the firearm rearward.
  • Recoil operation uses the rearward movement of parts of the weapon counter to the eject a moving forward, as described by Newton's third law of motion.
  • Gatling and other mechanical means utilize mechanical energy from an operator turning a crank.
  • Chain and others utilize external power through electrical or hydraulic energy for operation.


  1. U.S. Patent 1,907,163
  2. Smith, W.H.B.; Ezell, E. C. (1983), Small Arms of the World, 12th Edition, Stackpole Company, Harrisburg PA
  3. Major Thomas P. Ehrhart Increasing Small Arms Lethality in Afghanistan: Taking Back the Infantry Half-Kilometer. US Army. 2009
  4. Major Thomas P. Ehrhart Increasing Small Arms Lethality in Afghanistan: Taking Back the Infantry Half-Kilometer. US Army. 2009
  5. Major Thomas P. Ehrhart Increasing Small Arms Lethality in Afghanistan: Taking Back the Infantry Half-Kilometer. US Army. 2009
  6. U.S. Patent 2,090,656 Page 8, column 2, lines 67-70, Pg 9, column 1, lines 22-39
  7. Charles E. Petty, Delightful diversion: testing Kimber's new rimfire was a tough job, but someone had to do it, Guns Magazine, March, 2004. Contains some discussion about the floating chamber device.
  8. S. P. Fjestad (1991). Blue Book of Gun Values, 13th Ed.. p. 291. ISBN 0-9625943-4-2. 
  9. С.Л. Федосеев (2001), Оружие современной пехоты. Часть 1, Москва: ACT / Астрель. ISBN 5-17-008110-3, page 45
  • Hatcher, J. S. (1962). Hatcher's Notebook. Stackpole Books, ISBN 0-8117-0795-4
  • Smith, J.E.; Smith, W.H.B. (1960), Small Arms of the World, 6th Edition, Stackpole Company, Harrisburg PA
  • Smith, W.H.B.; Ezell, E. C. (1983), Small Arms of the World, 12th Edition, Stackpole Company, Harrisburg PA
  • Smith, W.H.B.; Smith, J.E. (1963), Book of Rifles, 3rd Edition, The Stackpole Company, Harrisburg PA
  • Balleisen, C.E. (1945).Principles of Firearms. John Wiley and Sons, Inc., New York NY
  • Chinn, G.M. (1955), The Machine Gun Volume IV, USGPO for the US Navy Bureau of Ordnance, Washington DC, pp. 130–134
  • Shalaby, S.H., "Automatic Weapon", Brassey's Encyclopedia of Land Forces and Warfare, 2000 Edition, Brassey's, ISBN 978-1-57488-087-8

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