More interest was shown in automatic cannon and two different types saw service; the 1½ Pr C.O.W. gun (so-called because it was developed by the Coventry Ordnance Works) and the 1 Pr Vickers Mk III. The predecessor of the C.O.W. gun – the 1 Pr – was actually test-fired from an F.E.3 in 1913 (although the plane was suspended from a gantry rather than flying) and was also intended for the F.E.6 of 1914, but probably never was fired in the air, and attention soon switched to the 1½ Pr. The appearance was distinctive in that the recoil spring was wrapped around the barrel in the interest of compactness. Ammunition was fed to the gun via a five-round clip, meaning that only a short burst could be fired before reloading (the cyclic rate of fire was 100-120 rpm). Development work took a long time and it was not until August 1917, following successful trials of the Mk.III gun, that an order was placed for 90 of these weapons; the maximum which could be produced without affecting the output of other guns. By August 1918 the orders had increased to 450 and the urgency of the order was stressed but problems continued and only fourteen had been delivered. The end of the war saw the cancellation of the orders with only those under construction being completed, and in all about 76 guns were delivered.
Only in October 1918 was a British machine available with a cockpit designed for the gun; it was fitted in the rear cockpit of three D.H.4 light bombers, angled upwards at 45º for use against the giant R-plane bombers as well as Zeppelins. A hole was cut into the upper wing for the gun to fire through. Aiming was the job of the pilot, who instructed the gunner/loader when to fire. Various mounting, handling, sighting and buffer problems were resolved and over 1,000 rounds fired but the muzzle blast caused problems and the plane required light alloy plating to protect it, as well as some structural strengthening. In the event, just three D.H.4 aircraft fitted with this gun entered service in November, and only two of these reached the front, but they had no time to see action before the Armistice.
The gun was also tested in the Tellier flying boat, a Voisin 8, two D.H.3A and a D.H.10 twin-engined bomber and was intended for the three-seat F.E.4 of 1916. Two C.O.W. guns were proposed in August 1918 as defensive armament for the big Handley-Page V/1500 bomber.
After the War, the C.O.W. gun was tried in various installations, especially in large flying boats in which the gun was intended primarily for anti-submarine work. The twin-engined Armstrong Whitworth Sinaia, the Short Cromarty and the experimental Vickers Valentia, all made in 1921, had provision for one in the nose. In the late 1920s the Blackburn Iris and Perth, similar three-engined biplanes built in small numbers, were also available with a C.O.W. gun in their bow position instead of the usual Lewis, and in 1932 the Short Sarafand had a similar installation.
Some landplanes were proposed as mounts for the big cannon. The Bristol Bagshot and Westland Westbury were twin-engined three-seat heavy fighters, designed to a 1924 specification for bomber-destroyers and based around the C.O.W. gun. The Bagshot was a high-wing monoplane, the Westbury a biplane, but both were armed with two flexibly-mounted cannon in nose and dorsal positions. The muzzle blast was recognised to be severe enough to damage any normal structure so strengthening had to be applied, and because of severe vibration on firing it was necessary to fit instruments and sensitive equipment on flexible mountings.
Perhaps the most remarkable were the two experimental C.O.W. gun fighters of 1931, designed to specification F.29/27 by Vickers and Westland. These single-seat, single-engined planes not only had a 37 mm C.O.W. gun as their sole armament, it was angled upwards at 45-55º for attacking bombers from below. The installation was based on the theory of the "no allowance angle," at which the body lift from the projectile would compensate for gravity drop over an important part of the trajectory.
Despite all of this work, the only significance the C.O.W. gun achieved was to be used as the starting point for the Vickers 40mm Class S design, after Vickers had taken over the Coventry Ordnance Works. The remaining C.O.W. guns saw out their days on AA mountings as airfield defence weapons (some mounted on lorries) in the Second World War. They were not finally declared obsolete until 1948.