The Lewis gun is a First World War-era light machine gun.
Designed privately in the United States though not adopted there, the design was finalised and mass-produced in the United Kingdom, and widely used by troops of the British Empire during the war.
It had a distinctive barrel cooling shroud (containing a finned, aluminium breech-to-muzzle heat sink to cool the gun barrel) and top-mounted pan magazine.
A predecessor to the Lewis gun incorporating the principles upon which it was based was designed by Austrian gun designer Ferdinand Mannlicher.
The Lewis gun was invented by U.S. Army colonel Isaac Newton Lewis in 1911, based on initial work by Samuel Maclean.
Despite its origins, the Lewis gun was not initially adopted by the U.S. military, most likely because of political differences between Lewis and General William Crozier, the chief of the Ordnance Department.
Lewis left the United States in 1913 and went to Belgium, where he established the Armes Automatique Lewis company in Liège to facilitate commercial production of the gun.
Lewis had been working closely with British arms manufacturer the Birmingham Small Arms Company Limited (BSA) in an effort to overcome some of the production difficulties of the weapon.
The Belgians bought a small number of Lewis guns in 1913, using the .303 British round and, in 1914, BSA purchased a licence to manufacture the Lewis machine gun in England.
Lewis and his factory moved to England before 1914, away from possible seizure in the event of a German invasion.
The onset of the First World War increased demand for the Lewis gun, and BSA began production, under the designation "Model 1914".
The design was officially approved for service on 15 October 1915 under the designation "Gun, Lewis, .303-cal."
No Lewis guns were produced in Belgium during the war; all manufacture was carried out by BSA in England and the Savage Arms Company in the US.
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 Lewis gun used a pan magazine holding 47 or 97 rounds. Pan magazines hold the ammunition nose-inwards toward the center, in a radial fan.
The Lewis magazine was 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 recoil 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 recoil spring had an adjustment device to alter the recoil resistance for variations in temperature and wear.
The Lewis design proved reliable and was even copied by the Japanese and used extensively by them during the Second World War.
The gun's cyclic rate of fire was about 500-600 rounds per minute.
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