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The Lewis gun holds the title of being the first practical light machine gun (disputed). Even though its looks don't suggest so, Lewis had been the ancestor of automatic weaponry nowadays.
Developed in 1911 by US Army Colonel Isaac Newton Lewis, it was a reworking of design by Samuel McLean - initial design was so cumbersomely weightened over with gadgets, that it provided no combat value.
Isaac Lewis took the initial design, stripped all surrounding gadgets, and redesigned the gun in quite straightforward manner. End result is well-known to everyone.
Lewis gun was mounted on the aeroplane, to demonstrate the viability of aircraft combat. However, US army had promptly rejected the whole idea, asserting that aeroplanes could not be viably used in warfare somehow else then in scouting role. (About two years later, this illusion had been quite violently disrupted.)
Later, around 1913, Lewis had been tried by army for the role of infantry machine-gun, and again, rejected. Isaac Lewis had retired from US army, and traveled to Europe shortly after, where in Belgium, he formed Armes Automatiques Lewis, which began the production of those guns in 1914. In the same time, Isaac Lewis had struck a deal with Birmingham Small Arms Co., granting license for Lewis machine-gun production.
Lewis guns were adapted in service in UK and Belgium, and widely exploited in World War 1. Germans, who suffered numerous casualties from this gun during air combat, nicknamed it "Belgian Rattlesnake". Loaded with incendiary bullets, Lewis machine-guns mounted on quick aeroplanes had proved to be lethal against hydrogen-filled German zeppelins. Germans instantly adapted every captured Lewis gun in service.
Reliable, cheap and simple, Lewis guns served valiantly through the World War 1, and were exported in various countries, including, ironically, USA. Among others, tsarist Russia had procured a big number of Lewis guns, which served on with Red Army until the introduction of Degtyarev DP-29.
In a letter from T.E. Lawrence to his friend and biographer, the poet, Robert Graves, Lawrence noted that he carried an "air-Lewis gun" in the saddle bucket on his camel during the war.
During World War 2, Lewis machine-guns saw action again, mainly in 1941–1942 years. Employed by UK, they were used to compliment the lack of Bren guns, and utilize stocks as fully, as possible.
Interestingly enough, the Mexican Mendoza RM-2 and German FG-42 seems to borrow heavily from Lewis design. In turn, M-60 borrows action from FG-42 - and this is only one of examples of Lewis technical solutions living on through generations of firearms.
Technically, Lewis guns are gas-operated, air-cooled weapons. "Stovepipe" barrel shroud is often mistaken for water cooling, but that is not so. Rather, Lewis guns have shroud project in front of the barrel, tapering in. This form utilises the discharge of powder gases during fire to forcibly draw air through the shroud, thus cooling the barrel. Some sources indicate, however, that this shroud was quite unnecessary.
Pan magazines were of two types - for 47 and 97 rounds. Big drums for 97 rounds were intended for the aircraft-installed guns, as they were quite sensitive to rough handling. Smaller 47-round drums were sturdier, but they were still quite sensitive to fouling, as their design had them open at the bottom.
Lewis guns had interesting technical solutions - pan was driven by gas-operated cam, rather than manually-wound spring. Another interesting feature was return spring, which was similar to clockwork spring, wound and unwound by another cam.
Lewis guns are considered to be among best guns of World War 1. Ideas incorporated in them were used in numerous other firearms. Arguably, Lewis guns are ancestors of whole class of light guns.