A muzzle booster or recoil booster is a device affixed 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. Muzzle boosters are usually used to improve the reliability and/or rate of fire of a recoil operated firearm. The muzzle booster is the antithesis of 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, and it does not alter the felt recoil of the weapon, it merely adds more energy to the operating components.
The recoil booster was first introduced in the Vickers machine gun of 1912. The Vickers, like the Maxim gun from which it was developed, uses a short-recoil action. At firing, the recoil from the cartridge pushes the 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.
A Vickers-type booster (the "typical" muzzle booster type) consists of two parts; the first is a flared "cup" on the muzzle of the barrel and the second is a perforated tube around the end of the muzzle, attached to the main body of the weapon, and with an end closed except for a small hole for the bullet to pass through. As the bullet exits the barrel, the expanding gases follow it into the chamber created between the cup and the shroud. As it passes through the close tolerance hole in the end of the perforated tube, it temporarily forms a blockage to further forward movement of the expanding gas from the barrel. The pressure inside the booster rises very rapidly as the gases continue to expand in the confined space (even after the bullet has cleared the hole, the gas pressure is still very high). 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 action — the barrel being acted upon as if it were the piston. Muzzle boosters in other weapons act in a similar fashion.
As one can see, 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. Thus, the name of "recoil booster" can be misleading. 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 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.
The original use of the recoil booster was to provide additional energy to move the large barrel/bolt mass on recoil operated machine guns. The primary German machine gun of WWI was the Maxim-based MG 08, a water-cooled heavy machine gun which used a muzzle booster to increase rate of fire and reliability. Like the Vickers, the barrel was surrounded by a heavy water-jacket for cooling, which 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-cooled 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 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 such as the lMG 08/15 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 amount of metal originally removed had been excessive. The World War II-era German MG42 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 MG3, essentially an MG42 modified to use the standard 7.62×51mm NATO round, and currently used by the German military, still uses a boosted short recoil design.
Recoil boosters have found a use in suppressors on short-recoil operated semi-automatic pistols (i.e., most modern centerfire pistols chambered for 9mm and above). Such pistols have a barrel and slide assembly which recoils a short distance upon firing before the barrel is unlocked from the slide (usually by tilting the barrel in order to separate the interlocking lugs on the barrel and the slide). Since the weapon relies on momentum to carry the slide all the way to the rear to operate the mechanism, and it is designed to work with a given weight of slide and barrel, and a given power of cartridge, adding the extra weight to the barrel by screwing on a suppressor will interfere with the gun's ability to properly cycle the action after each shot, resulting in stoppages. By incorporating a recoil booster (also known as a Nielsen device), the weight of the suppressor can be uncoupled from the barrel at the moment of firing, allowing the pistol to function properly by boosting the recoil energy of the barrel and slide, and by temporarily decreasing the effective attached weight. This is achieved by mounting the body of the silencer to a spring, which attaches to a piece that screws onto the muzzle of the firearm. When fired, the force of the gases expanding inside the baffles of the suppressor act to force the suppressor body forward in relation to the barrel. The spring allows the barrel to recoil while the suppressor stays in place; the pressure of the gases between the two components helps force them apart as well, much like the VIckers-type boosters described above. Many suppressor boosters incorporate an indexing system which allows the suppressor to be reoriented in a number of different rotational positions, allowing the end user to fine-tune the weapon's point of aim.
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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