Gun automatics: gas operated actions

Gas operated action, that is, automatic or self-loading gun systems that use the high pressure powder gases released by the exploding cartridge to cycle the action after each shot, are quite old. Therefore their maturity period was noticeably longer than the maturity period for most other successful automatic gun actions which utilise the recoil impulse to cycle the gun.

The key reason for the slow start was the nature of gun powder and its burning characteristics. Until the advent of smokeless powder, thick black powder residue tended to clog or jam even the most sturdy manually operated firearms (such as revolvers) after just a few dozen shots. Early smokeless powders produced noticeably less residue, but their internal ballistic properties varied widely depending on the powder type and batch, environmental conditions etc. Variations of the pressure at the gas port, drilled in the bore to bleed the gas to the operating piston, caused variations in the velocities and impulses of the moving parts, affecting reliability and the durability of the guns. Also, due to insufficient knowledge of the processes that happen inside the bore during discharge, many designers were afraid of gas port erosion, the decrease of muzzle energy (due to gas bleeding off from the bore) etc. As a result, gas operated systems became truly prolific only during the interwar period. By the end of WW2 gas operated actions became the preferred systems for most automatic long guns, thanks to advancements in metallurgy, propellant chemistry and internal ballistics science. Compared to recoil operated actions, most gas operated guns offered better accuracy due to the maintenance of a stationary barrel, and a somewhat lighter construction due to a lack of the necessity for a long barrel jacket. In fact, almost all WW2 era semi and fully automatic hand-held long guns that fired rifle or intermediate ammunition were of a gas operated design. All major self-loading and automatic rifles of the era (US M1 Garand and M1 Carbine, Russian Tokarev SVT-40, German G.41, G.43 and StG.44) were gas operated designs. In production numbers they outshadowed the only recoil-operated rifle, the US Johnson M1941, by more than 100:1 in magnitude. Today, gas operated actions dominate in most types of hand-held and mounted “long” small arms, gradually replacing the few older recoil-operated (MG-3 or Browning M2HB) or delayed blowback (HK G3 and HK33) weapons still in service. There are only two closely linked classes of automatic firearms where gas operated actions have very little influence – handguns and submachine guns. Both fire relatively low-pressure rounds from relatively short barrels; as of today, there’s only one production handgun (the famous and monstrous “Desert Eagle” by Magnum Research) and less than a half-dozen submachine guns that use any sort of gas operation (i.e. Type 79 from China). Most pistols use either a blowback system or a short recoil system; most submachine guns use simple blowback.

It is interesting to note that the basic principles of gas operated, locked breech firearms were first invented and patented before or shortly after the turn of the 20th century. One of the earliest gas operated designs belongs to the French brothers Clair, who patented a gas operated rifle as early as 1892. Their weapon featured a gas cylinder, located below the barrel and connected to the bore via a gas port. Upon discharge, hot powder gases are fed into the cylinder through said port, pushing the piston forward and thus compressing a strong action spring. Once this spring is fully compressed, the gas pressure is released and the power of the compressed spring is used to cycle open the action of the gun. This system, later repeated with a similar lack of success in several designs, sought smooth operation of the action regardless of the variations in the pressure and the violent nature of powder gases.

Clair brothers patent for a gas operated rifle, 1892
Clair brothers patent for a gas operated rifle, 1892


In 1895, John Browning patented his own version of a gas operated action. He also used a hole drilled in the barrel (his earliest experiments were conducted using muzzle blast), but his patent used an oscillating piston, which resulted in a gradual opening of the breech, thus helping to ensure slow initial extraction and avoid torn rims and split cases. It is interesting  also that his patent contained a simplified design with a blow-forward piston, but it was not used by the inventor for the reasons described above. In fact, Browning’s gas operated machine gun, produced by Colt as their Model 1895, was one of the first successful military gas operated weapons. Known by the Americans as the “potato digger” due to its piston swinging below the barrel and digging the dirt if its barrel was too close to the ground, this weapon served through several minor campaigns and one very large Great War.

John Browning's patent for a gas operated carbine that uses swinging piston below the barrel. This patent was later used as a base for Colt Browning M1895 machine gun
John Browning’s patent for a gas operated carbine that uses a swinging piston below the barrel. This patent was later used as a base for the Colt Browning M1895 Machine Gun


Apart from a machine gun, Browning also patented a semi-automatic gas operated pistol. It had a swinging piston as well,  this time located above the barrel. Hardly a practical gun, it was never put into production; unlike the much more famous blowback and short-recoil operated handguns from the same designer. At about the same time someone called Carl Ehbets, an employee of Colt Firearms Co. who worked closely with John Browning, also patented an interesting gas operated pistol. It used a gas cylinder below the barrel, connected to the bore of the gun, and a long stroke piston that moved rearwards under the pressure. Upon discharge, the rearward movement of the piston first unlocked the bolt, then cycled it back for reloading. Basically, it was as modern a system as it gets.

Karl Ehbets patent for a gas operated pistol from 1896.
Carl Ehbets’ patent for a gas operated pistol from 1896.


At this point we must divert a bit from our historical retrospective and discuss some basic types of gas operation. As mentioned above, the fundamental principle of this system is to utilise the pressure of the burning powder gases, usually operating a certain moving part (known as the “piston”). This in turn would transfer its movement to parts of the action to unlock and open the breech, eject the fired case in the process and feed in a fresh round. Closing the breech and locking it is usually conducted by a spring, compressed during the first part of the cycle.

Gas operated systems can be classified by the following attributes:

1. Location of the gas cylinder and piston. Most popular variations are:
A. Below, above or to the side of the barrel, with the gas cylinder running in parallel with the bore and connected to it through a gas port (a hole drilled in the side of the barrel).

B. Around the barrel, somewhere along its length or at the muzzle. In the first case, the gas cylinder is formed by the external surface of the barrel and enclosing tubular cylinder, and the gas piston has an annular shape. In this case the gas cylinder is connected to the bore via one or several gas ports. In the second case, there are two further variations. Either the gas cylinder is similar to the previous design but extends forward of the muzzle, capturing muzzle blast to push the annular piston rearwards, or the gas piston is a cup-shaped affair with small hole in its base for the passage of the bullet. In this case, muzzle blast pushes the cup-shaped gas piston forward.

C. There’s no separate or “dedicated” gas cylinder per se; hot powder gases are fed from the gas port in the bore and through the gas tube towards a bolt carrier, where they either directly impinge on the bolt carrier itself or are fed into the internals of the bolt group to expand there and force the bolt carrier rearwards against the temporarily stationary bolt.

2. Length of the piston stroke. Generally speaking, there are two systems, known as “short stroke piston” and “long stroke piston”.
With long stroke systems, the gas piston is rigidly attached to the breech block / bolt carrier and moves rearwards for the entire length of the action cycle (even if the actual length of the “active” part of the cycle when the gas is working is noticeably shorter).
In short stroke systems the gas piston is invariably a separate part (of a group of parts) that give the bolt group a short “tap” before stopping in its tracks and leaving the bolt group to complete its reloading cycle alone.

One of the less obvious benefits of a gas system that uses a gas port is that it can be easily adjusted for various environmental or other conditions. Changing the cross-section of the gas port directly effects the amount of hot powder gases allowed to act on the gas piston, thus varying the power available to cycle the gun. This can be used to ensure the reliable functioning of the gun under various conditions (i.e. to add power to the system if it is too clogged after prolonged use), or to vary the rate of fire (which is especially useful for machine guns). There are many variations of gas regulation systems that can offer anything between two and more than twenty settings, with some also offering a complete gas cut-off (useful for use with sound suppressors or with certain types of muzzle-launched rifle grenades). Some weapons feature various automated gas regulators that ensure reliable functioning with a wide variety of loads. This feature is quite useful in hunting shotguns, which therefore can be used with a broad variety of loads, from light “sporting” loads to heavy magnum loads containing slugs or shot. The simplest automatic gas regulator is usually made as a spring-loaded valve, which bleeds off to the atmosphere excessive powder gases. The higher the pressure inside the gas cylinder, the more gas is allowed to escape before effecting the gas piston.

It must be noted that despite a wide variety of gas systems designed and implemented over the last century and a quarter only a few have remained truly popular over the years. In regard to the placement of the gas piston, it is usually located above or below the barrel, most often opposite the feed system. That is, on most gas operated belt fed machine guns the belt feed unit is above the barrel and the gas cylinder is below; on most modern rifles the gas cylinder is at the top and the magazine is at the bottom. There are obvious exceptions to this rule, such as some of the early self-loading and automatic rifles (for instance the French RSC M1917, US M1 Garand and M14), as well as most gas operated shotguns (where annual gas piston is usually located around the magazine tube that runs below the barrel).

The use of a long or short stroke piston is also arbitrary; it is generally accepted that short stroke pistons are more conducive to accuracy – (good examples are the Dragunov SVD or HK 417 / G28 sniper rifles), while long stroke pistons are more reliable under harsh conditions (best examples are the Bren machine gun, M1 Garand and Kalashnikov AK).
Annular gas pistons are rarely used in anything except shotguns, which normally operate under lesser pressures compared to rifles. Muzzle cup actions are things of the past, because they require very long action rods and badly affect the rifles’ balance.
The last, but not least is the so called “direct impingement” gas operated action that uses no gas piston at all. Originally devised by French designers during late 1920s, this type of action fed the powder gases through the long gas tube directly to the forward face of the bolt carrier, which itself served as a piston. Originally used in the Swedish AG.42 and in French MAS-49 rifles, after WW2 this system was taken further by Eugene Stoner in his AR-10 , AR-15 and M16 rifles, which allowed the powder gases to enter the hollow bolt carrier body and expand inside, against the bolt head at the front and the inner bolt carrier wall at the rear. This system is especially conducive to accurate shooting, because of the symmetrical nature of the forces applied to the bolt group. It is however, also more sensitive to fouling and “dirty” powders than piston-operated actions.


patent for the direct impingement gas action for Swedish AG-42 rifle
patent for the direct impingement gas action for Swedish AG-42 rifle

early Stoner patent for his version of the direct impingement gas system, with gas tube running to the side of the barrel
early Stoner patent for his version of the direct impingement gas system, with gas tube running to the side of the barrel (as opposed to later systems with tube located above the barrel)


Long stroke gas operated systems are the oldest and probably the most widespread in the world. Starting with early guns such as the Hotchkiss M1899 machine gun, Lewis machine gun of 1912, French RSC M1917 semi-automatic rifle and Browning BAR M1918, this system was later used in many highly successful designs such as Czechoslovak ZB-26 light machine gun, US M1 Garand rifle and Kalashnikov AK assault rifle, along with its many descendants. A Long gas piston adds noticeable weight to the bolt group, slowing down its rearward movement due to initial inertia and then maintaining high reliability due to the same inertia once parts acquire some speed.

1896-dated patent for early version of the Hotchkiss machine gun, with nlong stroke gas operated action
1896-dated patent for early version of the Hotchkiss machine gun, with nlong stroke gas operated action

Garand's 1930 patent for his version of the long stroke system with piston located below the barrel
Garand’s 1930 patent for his version of the long stroke system with piston located below the barrel


Short stroke systems appeared “en masse” during the inter-war period in several semi-automatic rifles. One benefit this system offers for rifles is that the bolt carrier is not coupled to the action rod connected to the piston. That way, the conveniently located top piston does not interfere with the top loading of the rifle when using stripper clips, a feature preferred by many military “customers” up until the late fifties. To name a few of the most popular rifles that used short stroke action in this way: there were Russian Tokarev SVT-40 and Simonov SKS-45’s , German Kar.43‘s, Belgian FN-49‘s and the FN FAL. American M1 carbine and M14 rifle stood out a little separately there, as they have the gas piston located below the barrel. This results in an intricately bent action bar, similar in concept to that used in the M1 Garand rifle. Most modern rifles that use short action pistons, dispensed with clip loading and used top gas pistons with simple action rods that impinge on the front of the bolt carrier. Some prominent weapons of this type are for instance the US AR-18 rifle and many of its spiritual descendants, such as German HK G36 and HK 416, or British L85A1.

Short stroke gas piston system, patented in 1940 by David Williams, and later used in M1 carbine
Short stroke gas piston system, patented in 1940 by David Williams, and later used in M1 carbine

Patent for FN FAL rifle, showing simple short stroke gas piston rod in front of the separate bolt carrier.
Patent for FN FAL rifle, showing simple short stroke gas piston rod in front of the separate bolt carrier.

As we can see from the above discussion, gas operated systems now dominate the world of long guns, rifles, shotguns and machine guns. The one apparent rival in this field would be the inertia system, but that is only practically applicable for shotguns.