Lewis gun (or Lewis
automatic machine gun
or Lewis automatic rifle)
In service 1913–1942

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's aluminium barrel-shroud caused the muzzle blast to draw air over the barrel and cool it, due to the muzzle-to-breech, radially finned aluminium 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)^[18] 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 necessary: in the Second World War, many old aircraft guns that did not have the tubing were issued to anti-aircraft units of the British Home Guard and to British airfields,