A Lewis Gun, in .303 British, at 2000 frames per second:
The Lewis Gun was developed by American Isaac Newton Lewis shortly before WWI, but the US military was not interested in it (in part because of a bitter dislike between Lewis and the Chief of Army Ordnance Crozier). Lewis instead licensed production to BSA in England, and the gun because a major part of UK armament during the First World War, both in ground and aerial roles. For a more complete view of the Lewis, check out my previous video on it, with disassembly and shooting:
Updated to add:
As pointed out by Daniel Watters, General Crozier’s book about his work during WWI (which is largely a defense of his actions regarding the Lewis) is in the public domain, and can be read or downloaded free at Archive.org. A good way to see the other side of the story, for folks who are interested.