The COW 37 mm gun was a British automatic cannon that was developed as a heavy-calibre aircraft weapon. It was trialled in several installations and specified for the Westland C.O.W. Gun Fighter. However they did not yield results and the weapon did not enter general service.
Coventry Ordnance Works had been set up in 1905 by a consortium of British shipbuilding firms (John Brown, Cammell Laird and Fairfield) in order to compete with the duopoly of Vickers and Armstrong-Whitworth in producing naval guns. Besides the larger naval gun, COW worked at the smaller end on anti-aircraft guns. There was a demand for a gun that could be mounted on an aircraft. Their first attempt at an automatic gun was a "1-pounder" (the nominal weight of the shell) from a rimless 37x94 cartridge. This developed into a 1½-pounder using a longer 37x190 cartridge in a five-round clip. The gun was ready to produce only as the First World War came to an end and was only in service briefly fitted to a pair of Airco DH4s.
After Vickers acquired the Coventry Ordnance Works, the COW 37 mm was used as for the development of the 40 mm Vickers S gun which was used by Hawker Hurricanes as an anti-tank weapon.
In the Second World War, COW guns were used as the armament for the Mk III version of the Armadillo armoured fighting vehicle; the COW gun with its shield on the rear part of the flatbed. The vehicle was used by the RAF for airfield defence and later the Home Guard.