Recoil operation is an operating mechanism used in locked-breech, autoloading firearms. As the name implies, these actions use the force of recoil to provide energy to cycle the action. Other operating systems are Blow forward operated, blowback operated, gas operated, gatling, and chain.
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When the ejecta of a firearm (the projectile(s), propellant gas, wad, sabot, etc.) are accelerated down the barrel of a firearm, all or a portion of the firearm will begin to move in the opposite direction, as required for conservation of momentum, according to the formula:
which is calculated by:
In non-recoil-operated firearms, it is generally the entire firearm that recoils. However, in recoil-operated firearms, only a portion of the firearm recoils while inertia holds another portion motionless relative to a mass such as the ground, a ship's gun mount, or a human holding the firearm. The moving and the motionless masses are coupled by a spring that absorbs the recoil energy as it is compressed by the movement and then expands providing energy for the rest of the operating cycle.
Since there is a minimum momentum required to operate a recoil-operated firearm's action, the cartridge must generate sufficient recoil to provide that momentum. Therefore, recoil-operated firearms work best with a cartridge that yields a momentum approximately equal to that for which the mechanism was optimized. For example, the M1911 design with factory springs is optimized for a 230-grain (15 g) bullet at factory velocity. Changes in caliber or drastic changes in bullet weight and/or velocity require modifications to spring weight or slide mass to compensate.
Recoil-operated designs are broadly categorized by how the parts move under recoil.
Long recoil operation is found primarily in shotguns, particularly ones based on John Browning's Auto-5 action. In a long recoil action, the barrel and bolt remain locked together during recoil, compressing the recoil spring or springs. Following this rearward movement, the bolt locks to the rear and the barrel is forced forward by the spring. The bolt is held in position until the barrel returns completely forward during which time the spent cartridge has been extracted and ejected, and a new shell has been positioned from the magazine. The bolt is released and forced closed by its recoil spring, chambering a fresh round.
The long recoil system is over a century old and dominated the automatic shotgun market for more than half that century before it was supplanted by new gas-operated designs. While Browning halted production of the Auto-5 design in 1999, Franchi still makes a long-recoil–operated shotgun line, the AL-48, which shares both the original Browning action design, and the "humpbacked" appearance of the original Auto-5. Other weapons based on the Browning system were the Remington Model 8 semi-automatic rifle (1906), the Remington Model 11 & "The Sportsman" model (a model 11 with only a 2 shell magazine) shotguns, the Frommer Stop line of pistols (1907) and the Chauchat machine rifle (1915).
The short recoil action dominates the world of centerfire automatic pistols, being found in nearly all such weapons chambered for 9x19mm Parabellum or higher-powered cartridges (weaker cartridges, .380 ACP and below, generally use the blow-back method of operation). Short recoil operation differs from long recoil operation in that the barrel and slide recoil together only a short distance before they unlock and separate. The barrel stops quickly, and the slide continues rearward, compressing the recoil spring and performing the automated extraction and feeding process. During the last portion of its forward travel, the slide locks into the barrel and pushes the barrel back into battery.
The exact method of locking and unlocking the barrel is the primary differentiating factor in the wide array of short recoil designs. Most common are the John Browning tilting barrel designs, based either on the swinging link and locking lugs as used in the M1911 pistol, or the linkless cam design used in the Hi Power. Other common designs are the locking block design found in the Beretta 92, rollers in the MG42, or a rotating barrel used in the Steyr TMP among others. One unusual variant is the 1890 toggle bolt design of Hugo Borchardt, used in the German Luger pistol.
While the short recoil design is most common in pistols, the very first short-recoil–operated firearm was also the first machine gun, the Maxim gun. It used a toggle bolt similar to the one Borchardt later adapted to pistols. Vladimirov also used the short recoil principle in the Soviet KPV-14.5 heavy machine gun which has been in service with the russian military and middle eastern armed forces for 80 years. Melvin Johnson also used the short recoil principle in his M1941 rifle & M1944 light machine gun.
The newest design in recoil-operated firearms is the inertia operated system. In a reversal of the other designs, the inertia system uses nearly the entire firearm as the recoiling component, with only the bolt remaining stationary during firing. Because of this, the inertia system is only applied to heavily recoiling firearms, particularly shotguns. Currently the only inertia operated firearms are either made by Benelli, or use a design licensed from Benelli, such as Franchi.[1] In the Benelli implementation, a two part, rotating locking bolt, similar to that in many gas-operated firearms, is used as basis of the action.
Before firing, the bolt body is separated from the locked bolt head by a stiff spring. As the shotgun recoils after firing, inertia causes the bolt body to remain stationary while the recoiling gun and locked bolt head move rearward. This movement compresses the spring between the bolt head and bolt body, storing the energy required to cycle the action. Since the spring can only be compressed a certain amount, this limits the amount of force the spring can absorb, and provides an inherent level of self-regulation to the action, allowing a wide range of shotshells to be used, from standard to magnum loads, as long as they provide the minimum recoil level to compress the spring. Note that the shotgun must be free to recoil for this to work--the compressibility of the shooter's body is sufficient to allow this movement, but firing the shotgun from a secure position in a rest or with the stock against the ground will not allow it to recoil sufficiently to operate the mechanism. Likewise, weapons of this type must be modified (with the addition of extended magazines or stock saddled ammunition slings on shotguns, for example) with care, as any sizable increase in weapon mass can reduce the force of recoil below that required to cycle the action.
As the recoil spring returns to its uncompressed state, it pushes the bolt body backward with sufficient force to cycle the action. The bolt body unlocks and retracts the bolt head, extracts and ejects the cartridge, cocks the hammer, and compresses the return spring. Once the bolt reaches the end of its travel, the return spring provides the force to chamber the next round from the magazine, and lock the bolt closed.
Some short-recoil–operated firearms, such as the German MG42, use a mechanism at the muzzle to extract some energy from the escaping powder gases to push the barrel backwards, in addition to the recoil energy. This boost provides higher rates of fire and/or more reliable operation. This type of mechanism is also found in some suppressors used on short recoil firearms, under the name gas assist or Nielsen device, where it is used to compensate for the extra mass the suppressor adds to the recoiling parts both by providing a boost and decoupling some of the suppressor's mass from the firearm's recoiling parts.
Other autoloading systems are: