The Lewis Gun was a pre-WWI era British machine gun that continued to see service all the way through WWII. It is visually distinctive for its very wide diameter cooling shroud around the barrel, and for its top mounted drum magazines, which came in 50 and 90 round sizes. It was invented by an Americann army officer in 1911 but was never adopted by that country. It was designed with an aluminium barrel casing to use the muzzle blast to draw air into the gun and cool down the internal mechanism. It could fire 550 .303 rounds per minute. The gun weighed only about half as much as the monumental Vickers machine gun and was primarily chosen because it could be carried and used by a single soldier. It was also about 1/6 the cost of a Vickers, and was issued in droves to soldiers on the Western Front. In WW2 it was replaced by the Bren gun for most infantry uses, but the Lewis saw continued service as a vehicle mounted weapon, primarily as a side gunner's weapon on aircraft. Although it was probably obsolete for that role as well, but the British were facing something of a major economic crisis during the war, and had to use their existing stocks in whatever capacity made the most sense.
After WWII the Lewis was officially discontinued and all existing models were retired in favor of the Bren and other models.