Gas-operation is a 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.
In 1892 Browning licensed the patent to Colt, and by 1895, the design had been refined to produce the M1895 Colt-Browning machine gun. The baffle had become a piston, which was located in a cylinder near the middle of the barrel at the bottom. The cylinder connected to a small port in the barrel, which blew the piston, attached to a hinged arm, down and backwards. This energy was used to cycle the belt fed mechanism of the machine gun. While the swinging arm below the barrel caused some problems (it was nicknamed "potato digger" after its tendency to dig holes in the ground if fired from too low a position) it was adopted by a number of militaries around the world in calibers ranging from 6 mm Lee Navy to .30-40 Krag and .30-06 Springfield. The M1895 was made for export as well; the Russians ordered several thousand M1895 machine guns in 1914 for use in World War I, and it saw service in England, France, and various countries in South America.
A later variant of the M1895, commonly called the Marlin Gun after its builder, Marlin-Rockwell (now Marlin Firearms), replaced the swinging arm with a long cylinder and a piston attached directly to a the rod that operated the action. This version was designed for use on tanks and aircraft, and it remained in production through the end of World War I. The Marlin Gun's long cylinder design would later be used by Browning in the design of the M1918 Browning Automatic Rifle, and by John C. Garand in the final version of the M1 Garand.
Other builders began experimenting with gas operation soon after Browning. The French Manufacture d'armes de Saint-Étienne, or MAS, began experimenting with a new form in 1901 that eventually saw service as the MAS-49 rifle, and was copied by others, such as the Swedish Ag m/42. These rifles used a gas port located partway down the barrel, which is in turn connected to a tube that runs back to the bolt. This piston fits into a cylinder on the bolt carrier. While this uses the same short cylinder as the M1895, by applying the gas directly to the bolt carrier, the mass of the reciprocating parts is reduced significantly.
In 1931 inventor David Marshall Williams submitted a patent appliation for an automatic firearm which used a piston separate from, but acting on, the operating rod. This principle was used in the M1 carbine and variants. In 1956, Eugene Stoner patented a design that used a gas tube, similar to the MAS design, but rather than acting on the bolt carrier itself, it acted on a piston contained inside the bolt carrier. In this case, the bolt acts as a fixed piston, and the bolt carrier as a movable cylinder. Gas is routed into the bolt carrier, where it pushes upon the locked bolt, and in response the bolt carrier is pushed to the rear. This unlocks the bolt, disconnects the gas tube from the carrier, and the bolt and carrier continue travel rearwards to cycle the action. A rifle using Stoner's operating mechanism was eventually adopted by the US military as the M-16 rifle.
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", that moves only a fraction of an inch prior to stopping against 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. Most systems, however, vent excess gas into the atmosphere through slots, holes, or ports.
With a short-stroke or tappet system, the piston moves separate from the bolt group. It may directly push the bolt group parts as in the M1 Carbine or operate through a connecting rod or assembly as in the Armalite AR-18. 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. 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. These sudden stops disrupt the weapon's point of aim, espeicially with light weapons in full-automatic fire. The primary disadvantage of this system is that it relies more heavily on spring pressure rather than kinetic energy to chamber a round and lock the breech.
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, AK47, 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. The primary disadvantage to this system is the disruption of the point of aim due to the center of balance changing during the action cycle and energetic and abrupt stops at the beginning and end of bolt carrier travel.
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.
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