Lewis Gun - Design Details

Design Details

The Lewis Gun was gas operated. A portion of the expanding propellant gas was tapped off from the barrel, driving a piston to the rear against a spring. The piston was fitted with a vertical post at its rear which rode in a helical cam track in the bolt, rotating it at the end of its travel nearest the breech. This allowed the three locking lugs at the rear of the bolt to engage in recesses in the gun's body to lock it into place. The post also carried a fixed firing pin, which protruded through an aperture in the front of the bolt, firing the next round at the foremost part of the piston's travel.

The gun was designed with an aluminium barrel-casing which used the muzzle blast to draw air into the gun and cool down the internal mechanism. There is some discussion over whether the cooling tube was effective or even necessary—in the Second World War many old aircraft guns which did not have the tubing, were issued to anti-aircraft units of the British Home Guard and to British airfields. Other weapons were used on vehicle mounts in the Western Desert and did not suffer without the tube. They were found to function properly without it, leading to the suggestion that Lewis had insisted on the cooling arrangement largely to show that his design was different from Maclean's earlier prototypes. Only the Royal Navy retained the tube on their deck-mounted AA-configuration Lewis Guns.

The Lewis Gun utilised two different drum magazines, one holding 47 rounds, the other 97. Unlike other designs, the Lewis's drum was not wound against a spring but was mechanically driven by a cam on top of the bolt which operated a pawl mechanism via a lever.

An interesting point of the design was that it did not use a traditional helical coiled spring, but used a spiral spring, much like a large clock spring, in a semicircular housing just in front of the trigger. The operating rod had a toothed underside, which engaged with a cog which wound the spring. When the gun fired, the bolt recoiled and the cog was turned, tightening the spring until the resistance of the spring had reached the recoil force of the bolt assembly. At that moment, as the gas pressure in the breech fell, the spring unwound, turning the cog, which, in turn, wound the operating rod forward for the next round. As with a clock spring, the Lewis Gun's recoil spring had an adjustment device to adjust the recoil resistance for variations in temperature and wear. Unusual as it seems, the Lewis design proved enduringly reliable, and was even copied by the Japanese and used extensively by them during World War II.

The gun's cyclic rate of fire was approximately 500–600 rounds per minute. It weighed 28 lb (12.7 kg), only about half as much as a typical medium machine gun of the era, such as the Vickers, and was chosen in part because, being more portable than a heavy machine gun, it could be carried and used by a single soldier. BSA even produced at least one model (the "B.S.A. Light Infantry Pattern Lewis Gun", which lacked the aluminium barrel shroud and had a wooden foregrip) designed as a form of assault weapon.