The Lee Enfield Mark III* Sawn-Off is God's blessing to the insane.
It is essentially a magazine-fed single action revolver in .303 British with an effective range equal to one's arm lenght. The gun has no iron sights aside from a notch carved in the back of the stripper-clip feeding bridge, and is the only SMLE in history to actually make use of the detachable box magazine.
Although it makes use of the SMLE's impressive 10 round magazine, 8 will disappear, 1 will hitmarker (Somehow), and 1 will actually kill.
No one knows who took to making this abomination. They say the man who did it was lined up and shot dead by firing squad. Some men who have taken to wielding it have gone mad. They shun themselves from the rest of their squads or run out into no-mans-land never to be seen again. But, every morning no matter what has happened to the man wielding it, the Sawn-Off returns to its original trench waiting for its next victim.