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A muzzle booster or recoil booster is a device fixed to the muzzle of a firearm, intended to harness the energy of the escaping propellant to augment the force of recoil on portions of the firearm. In spite of its name, a muzzle booster does not increase muzzle force or velocity but instead is usually used to improve the reliability and/or rate of fire of a recoil operation firearm. It was invented by Hiram Maxim in 1894.
The muzzle booster is distinct from the muzzle brake, which is designed to use the propellant gases to reduce the recoil of the firearm. However, unlike a muzzle brake, a muzzle booster uses the pressure of the expanding gases rather than the reaction force. A muzzle booster does not alter the felt recoil of the weapon; it merely adds energy to the operating components.
At firing, the recoil from the cartridge pushes the Gun barrel and bolt together backwards within the gun. This movement provides the energy required to extract and eject the spent cartridge, and compresses the recoil spring to complete the cycle. The muzzle booster increased the recoil force transmitted to the barrel by directing some of the escaping gas into pushing the barrel back rather than letting it all expand outwards at the muzzle, in essence acting as an auxiliary gas-operating system, with the barrel acting the role of the operating rod. This increased the initial velocity of the barrel and bolt, providing more energy for the operation of the mechanism.
The cup on the muzzle of the barrel provides a large, movable surface for the gas to push against, as it exerts force equally in all directions. As the outer shroud is fixed to the main frame of the gun, and only the barrel is movable, the pressure forces the cup and barrel to the rear, acting exactly as a piston in a cylinder. As the barrel cycles to the rear, the cup passes the perforations in the outer shroud, opening an escape path for the gases, immediately lowering the pressure, both reducing the harsh impact when the barrel reaches full recoil and preventing the remaining pressure from the gases from acting as a "spring" and slowing the barrel when it begins to travel forwards again (which would only slow the rate of fire). The barrel continues to the rear on its own momentum, and actuates the operating mechanism. The resulting action can be seen as a composite of the recoil action and a gas operation — the barrel being acted upon as if it were the piston. Muzzle boosters in other weapons act in a similar fashion.
The name of "muzzle booster" can be misleading. The pressure within the muzzle shroud is exerted equally in all directions, pushing forwards on the shroud with the same force as it pushes the muzzle cup and barrel to the rear; thus, the actual felt recoil of the weapon is not increased, even though the force imparted to the operating system is.
It is unlike a muzzle brake in that the muzzle booster extracts work from the pressure of the expanding gasses, while a muzzle brake relies on redirecting the reaction force of the fast moving gases at the muzzle. In fact, by trapping and slowing, and then redirecting the gases sideways, the device also called a "recoil booster" actually somewhat reduces the felt recoil of the weapon, by eliminating the reaction component of expanding gases escaping the muzzle in a forward direction.
At the start of WWI the primary German machine gun was the Maxim gun-based MG 08, a Water cooling heavy machine gun equipped with a blank firing adapter but not a muzzle booster. Because of that it was inferior to similar British and Russian analogs with recoil boosters in its rate of fire and reliability, and by 1915 Germans introduced their booster designated Rückstossverstärker 08 S, increasing the rate of fire by about 50%.
The cooling jacket for all those designs also acted as the frame within which the barrel recoiled on firing, and which the fixed portion of the muzzle booster was mounted on. In 1915 an Air cooling version was created for use as a fixed aircraft gun, designated the lMG 08 (or LMG 08, traditionally with a lower case "L"). Eliminating water cooling saved a great deal of weight, but the water jacket was a crucial component of the gun, as it held the bushing which supported the muzzle end of the barrel and allowed it to recoil. Thus, the water jacket structure was retained, but was heavily perforated to allow cooling air-flow to reach the barrel, leaving more open space than metal. This much-lightened structure, resulting from just over 50% of the jacket's circumferential sheetmetal removed for its cooling slots; was strong enough to support the barrel, but not enough to handle the powerful forward counter-force created when the expanding gases in a muzzle brake forced the barrel rearwards. Thus, early versions of the lMG 08 deleted the muzzle booster, although later versions of the lMG 08, and its replacement, the lightened-receiver LMG 08/15 model which reduced the cooling barrel's diameter to just 92.5 mm, changed to a less-heavily perforated barrel shroud which could handle a muzzle booster, as it was realized that with the airflow over an aircraft-mounted machine gun, the fifty-percent-plus amount originally removed had been excessive.
The World War II-era Germany MG 42 was another such machine gun which also made use of a muzzle booster. This application has largely fallen out of use as modern machine gun design switched to delayed blowback and gas operation in many cases. The Rheinmetall MG 3, essentially an MG 42 modified to use the standard 7.62×51mm NATO round, and currently used by the German military, still uses a boosted short recoil design.
Some blank firing adapters (BFAs) act as recoil boosters, harnessing the gases produced by the blank cartridge to compensate for the lack of back pressure that would normally occur as the bullet traps the expanding propellant gases until the bullet clears the barrel. This is important on gas-operated firearms, as they rely on tapping some of the pressure built up behind an accelerating bullet moving up the barrel. Without a bullet to act as a "plug", the gases simply rush out the muzzle without generating enough pressure to cycle the weapon properly, since such devices are often fine-tuned to work only within a certain limited range of gas pressures.
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