Nielsen device

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.

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Wikipedia