page_title: Lewis gun
section_title: Design details
hierarchical_section_title: Lewis gun / Design details
caption_attribution_description: Lewis gun displayed in Elgin Military Museum in St. Thomas, Ontario
context_page_description: The Lewis gun is a First World War–era light machine gun. Designed privately in America but not adopted, 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 and top-mounted pan magazine. The Lewis served to the end of the Korean War. It was also widely used as an aircraft machine gun, almost always with the cooling shroud removed, during both World Wars.
context_section_description: 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's aluminum barrel-shroud caused the muzzle blast to draw air over the barrel and cool it, due to the muzzle-to-breech, radially finned aluminum heat sink within the shroud's barrel, and protruding behind the shroud's aft end, running lengthwise in contact with the gun barrel (somewhat like the later American M1917/18 Marlin-Rockwell machine gun's similar gun barrel cooling design) from the "bottleneck" near the shroud's muzzle end and protruding externally behind the shroud's rear end. Some discussion occurred over whether the shroud was really necessary—in the Second World War, many old aircraft guns that did not have the tubing were issued to antiaircraft units of the British Home Guard and to British airfields, and others were used on vehicle mounts in the Western Desert; all were found to function properly without it, which led 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/heatsink cooling system on their deck-mounted AA-configuration Lewis guns.
  The Lewis gun used a pan magazine holding 47 or 97 rounds. Pan magazines hold the rounds, bullet-noses inwards toward the center, in a radial fan. Unlike the more common drum magazines, which hold the rounds parallel to the axis and are fed by spring tension, pan magazines are mechanically indexed. 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. Unusual as it seems, 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. 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 machine gun, and was chosen in part because, being more portable than a heavy machine gun, it could be carried and used by one 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 fore grip) designed as a form of assault rifle.
caption: A Lewis gun at the Elgin Military Museum Canada. The rear end of finned aluminum heat sink, that fits within the gun's cooling shroud, can be seen
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