Most current gas systems employ some type of piston. The face of the piston is acted upon by combustion gas from a port in the barrel or a trap at the muzzle. Early guns, such as Browning's "flapper" prototype, the
Bang rifle, and the
Garand rifle, used relatively low-pressure gas from at or near the
muzzle. This, combined with larger operating parts, reduced the strain on the mechanism. To simplify and lighten the firearm, gas from nearer the
chamber needed to be used. This high-
pressure gas has sufficient force to destroy a firearm unless it is regulated somehow. Most gas-operated firearms rely on tuning the gas port size, mass of operating parts, and spring pressures to function. Several other 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. This mechanism inherently limits the amount of gas taken from the
barrel. 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. Most systems, however, vent excess gas into the atmosphere through slots, holes, or ports.
Gas trap A gas trap system involves "trapping" combustion gas as it leaves the muzzle. This gas impinges on a surface that converts the energy to motion that, in turn, cycles the action of the firearm. As the resulting motion is forward toward the muzzle of the gun, some sort of mechanical system is needed to translate this into the rearward motion needed to operate the bolt. This adds to the complexity of the mechanism and its weight, and the placement of the trap generally results in a longer weapon and allows dirt to easily enter the mechanism. Despite these disadvantages, they use relatively low pressure gas and do not require a hole in the barrel, which made them attractive in early designs. The system is no longer used in modern weapons. In 1884,
Hiram Maxim patented a muzzle-cup system described in , though it is unknown if this firearm was ever prototyped.
John Browning used gas trapped at the muzzle to operate a "flapper" in the earliest prototype gas-operated firearm described in and used a slight variation of this design on the
M1895 Colt–Browning machine gun "potato digger". The Danish
Bang rifle used a muzzle cup blown forward by muzzle gas to operate the action through transfer bars and leverage. Other gas-trap rifles were early production
M1 Garands and German
Gewehr 41 (both Walther and Mauser models). The American and German governments both had requirements that their guns operate without a hole being drilled in the barrel. Both governments would first adopt weapons and later abandon the concept. Most earlier US M1 Garand rifles were retrofitted with long-stroke gas pistons, making the surviving gas trap rifles valuable on the collector's market. In the 1980s, Soviet designer Alexander Adov from
TsKIB SOO modified the concept with a tube diverting gas from the muzzle to a standard long-stroke system (see below) in order to diminish influence of the gas engine on barrel and increase accuracy, but his sniper rifle wasn't adopted due to the
dissolution of the Soviet Union.
Long-stroke The stroke length of a gas-operated and piston-actuated firearm is defined by the distance travelled (by the piston) in relation to the major diameter of the piston itself. Specifically in relation to long-stroke mechanisms, the travel distance of the piston while under pressurised propulsion must be greater than the major diameter of that piston; this distance must be reached prior to the ventilation of the propulsive gasses. If the pressurised propulsive gasses are ventilated prior to this point, this would define a short-stroke piston-actuated mechanism. An often-cited example of a long-stroke gas-operated system is that where the form and fit must involve the piston, operating rod, and bolt carrier group fixed together to form a single unitary assembly. This is not a factually correct definition and is simply an observation of paired yet unrelated features within various firearms designs. It is for this reason that true long-stroke piston-actuated firearms are rare, and why such misconceptions and distortions of definitions arise. Examples of long-stroke piston-actuated firearms include (but are not limited to) the M1 Garand, and the AKM. Other firearms, such as the M1918 BAR, present an interesting example of a firearm which could be defined as either a short-stroke or a long-stroke mechanism. The principal distance travelled by the piston while under pressurised propulsion is less than that of the diameter of the piston itself. However, the ventilation ports are located sufficiently rearwards so that a distance travelled by the piston while under some meaningful pressurisation is greater than that of the piston’s diameter. Which in effect renders the M1918 BAR’s mechanism a hybrid. The pistons initial travel is fully pressurised for a shorter distance, the gas regulator body then abruptly end where the gasses can partially bypass the piston while still providing direct propulsion at a lower pressure. After achieving a stroke length greater than the piston diameter, the gasses escape through the six consecutive (one row of three on each side) ventilation ports within the gas cylinder tube.
Short-stroke In contrast to the long-stroke mechanism described above, a short-stroke piston-actuated mechanism is defined by a gas piston that travels under pressurized propulsion for a distance less than its own major diameter. The propellant gases are vented or bypassed before the piston reaches a distance of travel equal to its diameter. Short-stroke gas systems are more common than long-stroke systems and include sub-variants such as the tappet system found in the Steyr AUG; a tappet mechanism is not mechanically linked to the bolt carrier group and is often the cause of the misconception of what a short-stroke mechanism truly is. As with long-stroke systems, the distinction between short- and long-stroke mechanisms is often misunderstood. A frequently repeated misconception is that a short-stroke system must feature a piston and operating rod that are not connected to the bolt carrier group (a tappet mechanism). While this is a common feature in many designs, it is not a defining characteristic of stroke length. The classification depends solely on the distance the piston travels under pressure in relation to its diameter, not on whether the piston is physically linked to the rest of the operating assembly. Returning to the M1918 BAR example previously discussed in the context of long-stroke mechanisms, its classification can also support a short-stroke interpretation. The BAR’s gas regulator includes a 0.035-inch diameter orifice that vents gas forward through the regulator body. This continuous gas venting from the outset limits the duration and distance over which the piston experiences peak pressurization. As a result, even though the piston and operating rod move as one, the piston itself may not remain under full pressure long enough to qualify as a true long-stroke system—thereby classifying the M1918 BAR as a hybrid or borderline short-stroke mechanism. It is for the aforementioned reasons that stroke length, when used to describe the technical operation of firearm components—as in the contexts of marketing, patent filings, or technical documentation—should not be applied arbitrarily or based solely on visual or structural features. Instead, it should reflect the actual pressure-driven stroke distance of the piston in relation to its geometry.
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,
Steyr GB and
Walther CCP pistols.
Floating chamber To avoid consuming a lot of relatively expensive rounds, many armies, including the United States Army,
trained machine gun crews with less-expensive sub-caliber ammunition in the late 19th century and the first half of the 20th century. To do this, they needed a cheap
.22 LR 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. The .22 caliber
Colt Service Ace conversion kit for the .45 caliber
M1911 pistol also used Williams' system, which allows a much heavier slide than other conversions operating on the unaugmented
blowback mechanism and makes training with the converted pistol realistic. A floating chamber provides additional force to operate the heavier slide, providing a felt
recoil level similar to that of a full power cartridge.
Primer actuated Primer actuated firearms use the energy of
primer setback to unlock and cycle the firearm.
John Garand developed the system in an unsuccessful bid to replace the M1903 bolt-action rifle in the early 1920s. Garand's prototypes worked well with US military .30-06 ammunition and uncrimped primers, but then the military changed from a fast burning gunpowder to a progressive burning Improved Military Rifle (IMR) powder. The slower pressure rise made the primer actuated prototypes unreliable, so Garand abandoned the design for a gas operated rifle that became the
M1 Garand. AAI Corporation used a primer piston in a rifle submitted for the SPIW competition. Other firearms to use this system were the Roth machine gun, the
Postnikov APT assault rifle and Clarke carbine as described in . And in some cases as in the whole primer ejecting from the cartridge through a port in the bolt cocking the striker A similar system is used in the spotting rifles on the
LAW 80 and
Shoulder-launched Multipurpose Assault Weapon use a 9mm,
.308 Winchester based cartridge with a
.22 Hornet blank cartridge in place of the primer. Upon firing, the Hornet case sets back a short distance, unlocking the action.
Direct impingement The
direct impingement (DI) method of operation vents gas from partway down the barrel through a tube to the working parts of a rifle where they directly impinge on the bolt carrier. This results in a simpler, lighter mechanism. Firearms that use this system include the French
MAS-40 from 1940, the Swedish
Ag m/42 from 1942. The Stoner gas system of the American
M16, M4, and AR-15 style rifles utilize a modified version of this where a gas tube delivers gas into the bolt carrier to impinge on the bolt, which acts as a piston to cycle the rifle. One principal advantage is that the moving parts are placed in-line with the bore axis meaning that sight picture is not disturbed as much. This offers a particular advantage for fully automatic mechanisms. It has the disadvantage of the high-temperature
propellant gas (and the accompanying fouling) being blown directly into the action parts. Direct impingement operation increases the amount of heat that is deposited in the receiver while firing, which can burn off and cover up lubricants. The bolt, extractor, ejector, pins, and springs are also heated by the same high-temperature gas. These combined factors reduce service life of these parts, reliability, and
mean time between failures. == Other uses of gas in firearms ==