Ordnance, QF, 3.7in Gun

The Ordnance, QF, 3.7in Gun was the standard British medium anti-aircraft gun of the Second World War and was an advanced design that kept on being improved, ending the war as a radar controlled gun with automatic fuse setting and ramming equipment, making it vastly more effective in 1945 than in 1940.

After the First World War the only anti-aircraft gun left in British service was the Ordnance QF 3in 20-cwt, which dated back to 1914. It wasn’t until 1928 that a new specification was agreed, for a 3.7in gun firing a 25lb shell with a ceiling of 28,000ft. The new gun was expected to be mobile, and was to have a travelling weight of 8 tons, a road speed of 25mph and take 15 minutes to get into action.

3.7in anti-aircraft guns firing victory salute

After that nothing happened until 1934 when money was found to develop the new gun. Woolwich Arsenal and Vickers-Armstrong both produced designs, and the Vickers one was chosen. The first pilot model passed its acceptance tests in April 1936. Production began in April 1937 and the first twenty-two service guns were issued in January 1938. At this stage the plan had been to produce all of the 3.7in guns on mobile mounts, and the heavier 4.5in gun on fixed mounts. However a shortage of anti-aircraft guns meant that in November 1937 it was decided to accept 120 3.7in guns on easier to build static mounts. These guns could be placed on emplacements being built for 4.5in guns, and in theory be returned to the mobile role once enough 4.5in guns were ready. This never happened, and none of the 3.7in guns placed on static mounts were converted into mobile ones. Indeed by the end of its production run more 3.7in guns were built on static mounts than on mobile ones.

QF 3.7in AA Gun on the Ledo Road

The gun remained in production throughout the Second World War, averaging an impressive 228 guns per month over eight years.

The 3.7in gun had a horizontal sliding breech. The recoil mechanism was carried above the barrel.

The new gun was designed to use the most modern features. It was always expected to work alongside predictors, advanced gunnery computers that would calculate the range, speed and direction of the target aircraft. This information would be passed electronically to the gun and displayed on dials. The gun layer’s role was to operate the gun’s controls so it’s own dials matched the incoming data. As a result it was only built with very rudimentary emergency gun sights. This made it a much more effective anti-aircraft gun, but also meant that it couldn’t easily be pressed into anti-tank duties like the famous similar calibre German 88mm.

3.7in AA guns fire on V-1 Flying Bombs

The 3.7in gun came in a number of different static or mobile versions. The mobile versions needed outriggers to keep it stable.

These mountings also carried an increasing amount of advanced technology, so the static Mounting Mark 2C came with remote power control, fuse setting and a ramming machine.

The 3.7in gun became increasingly effective as the war went on. In the late 1930s it had an effective ceiling of 23,500ft firing shells with a powder burning Fuse 199 (defined as the height at which a target approaching at 400mph could be engaged for 20 seconds before the gun reaches 70 degrees elevation). The introduction of a mechanical time Fuse 208 increased this to 24,600ft (the old fuse could only burn for 30 seconds, the new fuse could run for up to 43 seconds). An improved Predictor No 2 combined with the mechanical fuse 208 raised this to 25,300ft. An improvement to the predictor raised this to 29,400ft. Finally the electronic predictor No.11 with Fuse 208 raised this again to 32,000ft.

The rate of fire also improved. When the gun was first introduced the fuse had to be set by hand, and the shell then loaded manually into the breech, producing a rate of fire of eight rounds per minute. In 1939 a Machine, Fuze Setting device was introduced. This had a dial which matched that on the predictor. The nose of the shell went into the MFS which set the fuse, which then had to be manually loaded.

Night view of 3.7in AA gun during Flying Bomb campaign

The biggest improvement came with the MFS No.11 of 1942. This was attached to the gun cradle and combined a fuse setting machine and loading tray. The shell was dropped into the loading tray, and a mechanical process then pushed it forward into the fuse setter, then withdrew it and swung it into place behind the breech, where it could be rammed home. This increased the rate of fire to 19 rounds per minute and also improved accuracy by making the time taken from setting the fuse to firing more predictable.

By the end of the war things had progressed even more, and most guns had remote power controls, so radar would provide data directly to the predictor, which would control and fire the gun, leaving the crew to load and clean the guns.

QF 3.7in anti-aircraft gun on ground support, Normandy

The improvement in effectiveness of this gun during the war is truly remarkable. During the night blitz of 1940 it was calculated that it took 18,500 shots to shoot down a single German aircraft. Bu 1944-45 it was only taking 150 shots to shoot down a V-1 flying bomb! 

A number of 3.7in guns went to France with the BEF in 1939. During the fighting in 1940 they rarely had the chance to prove themselves in the AA role, as high level attacks were rare, and they weren’t designed for use against low flying aircraft. However they did prove themselves to be useful anti-tank guns, including one incident where one gun destroyed two attacking German tanks with its gun lowered almost to its lowest elevation.

During the battle of Britain new German targets were quickly revealed, increasing the drain on the limited number of mobile 3.7in guns. As a result General Pile, commander of AA Command, ordered more sites for static 3.7in guns to be built, including at places where they weren’t currently needed, so that if they were called for the mobile guns weren’t the only option.

The 3.7in gun was used just about everywhere the British fought during the Second World War, with mobile guns seeing action in the Far East, North Africa, Italy, Normandy and many other campaigns. They were often used as standard artillery, especially later in the war when the threat from the Luftwaffe was greatly diminished.

3.7in AA gun bombarding Gothic Line

In 1942 work began on converting the 4.5in gun into a high velocity 3.7in gun. This entered service as the 3.7in Mk VI, and despite sharing the same calibre had little in common with the original 3.7in guns. By the end of the war most of the 3.7in guns still in use with AA Command were the Mk VI, which had a much higher effective ceiling than the original 3.7in guns.

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