QF 3.7-inch AA gun

During World War I, anti-aircraft guns and anti-aircraft gunnery developed rapidly. The British Army eventually adopted the QF 3-inch 20 cwt as the most commonly used type. Shortly before the end of the war, a new QF 3.6 inch gun was accepted for service but the end of the war meant it did not enter production. After the war, all anti-aircraft guns except the three-inch gun were scrapped.

However, the war had shown the possibilities and potential for air attack and lessons had been learned. The British had used AA guns in most theatres in daylight, as well as against night attacks at home. They had also formed an AA Experimental Section during the war and accumulated much data that was subjected to extensive analysis. After an immediate post-war hiatus, the army re-established peacetime anti-aircraft units in 1922. In 1925, the RAF established a new command, Air Defence of Great Britain, and the Royal Artillery's anti-aircraft units were placed under its command.

In 1924–5, the war office published the two-volume Textbook of Anti-Aircraft Gunnery. It included five key recommendations for heavy anti-aircraft (HAA) guns:

• Shells of improved ballistic shape with HE fillings and mechanical time fuses
• Higher rates of fire assisted by automation
• Height finding by long-base optical rangefinders
• Centralised control of fire on each gun position, directed by tachymetric instruments, which incorporated the facility to apply corrections of the moment for meteorological and wear factors
• More accurate sound-location for the direction of searchlights and to provide plots for barrage fire

Two assumptions underpinned the British approach to HAA fire. First, aimed fire was the primary method and this was enabled by predicting gun data from visually tracking the target with continuous height and range input. Second, that the target would maintain a steady course, speed and height. Heavy anti-aircraft units were to engage targets up to 24,000 feet (7,300 m). Mechanical, as opposed to igniferous, time fuses were required because the speed of powder burning varied with height so fuse length was not a simple function of time of flight. Automated fire ensured a constant rate of fire that made it easier to predict where each shell should be individually aimed.[5]

During the 1920s, Vickers developed the Vickers range clock (Predictor No 1), an electro-mechanical computer that took height and range data from an optical rangefinder, applied corrections for non-standard conditions and was used by its operators to visually track a target, its output predicted firing data and fuse setting via the "mag-slip" electrical induction system to dials on each gun in a battery, the gun layers moved the gun to match pointers on the dials. The three-inch AA guns were modified accordingly.[5]

In 1928, the general characteristics for a new HAA gun were agreed, a bore of 3.7 inches (94 mm) firing 25 lb (11 kg) shells with a ceiling of 28,000 feet (8,500 m). Financial stringency led to no action being taken until the 1930s, when the specification was enhanced to a 28 lb (13 kg) shell, 3,000 ft/s (910 m/s) muzzle velocity, a 35,000 feet (11,000 m) ceiling, a towed road speed of 25 mph (40 km/h), maximum weight of eight tons and an into action time of 15 minutes.

In 1934, Vickers-Armstrongs produced a mock-up and proceeded to develop prototypes of the weapon, which was selected over a competitor by the state Design Department and passed acceptance tests in 1936.[6][2][7] The weight specification was exceeded, the muzzle velocity not achieved and the mechanical time fuse, No. 206, was still some years from production. The igniferous No. 199 had to be used and its lesser running time limited the effective ceiling. The gun passed the trials by April of the following year,[6] and the gun production started later in the year.

On 1 January 1938, the British air defences had only 180 anti-aircraft guns larger than 50 mm and most of these were the older 3-inch guns. This number increased to 341 by the September 1938 (Munich Crisis), to 540 in September 1939 (declaration of war), and to 1,140 during the Battle of Britain. Production continued until 1945, averaging 228 guns per month throughout the period. Guns were also manufactured in Australia.

Being a high-velocity gun, with a single charge and firing substantial quantities of ammunition, meant that barrel life could be short and by the end of 1940 there was a barrel shortage. Some of the substantial numbers of spare barrels required were produced in Canada.

In British service, the gun replaced the 3-inch AA gun in HAA batteries and regiments of the Royal Artillery (RA), usually grouped into specialist AA brigades of Anti-Aircraft Command or the field armies. Each regiment usually had three batteries, each of eight guns in two troops. Over 160 of these HAA regiments, RA, plus five of the West African Artillery and two each for the Royal Marines, Hong Kong-Singapore Artillery, Royal Malta Artillery and East African Artillery were eventually formed.[8] Other World War II users were India (about 14 regiments), Canada (two or three regiments) and Australia (equivalent of about 13 regiments).

A static 3.7-inch gun assigned to the 127th Heavy Anti-Aircraft Regiment, mounted on a Pile platform at Orford, Suffolk, October 1944

Two versions of the gun were produced. One used a travelling carriage, for use by batteries in the field army. This consisted of a wheeled carriage (Carriage Mk I or Mk III) with four folding outrigger trails and levelling jacks. The wheels were lifted off the ground or removed when the gun was brought into action.

The other used a travelling platform (Mounting Mk II) with detachable wheels for guns to be used in static positions but which could be re-positioned. The mounting had a pedestal that was fixed to a solidly constructed, preferably concrete, platform on the ground. In 1944, it was found that a temporary platform built from railway sleepers and rails was adequate for the static guns, making them considerably easier to re-deploy without the cost and delay of constructing new concrete platforms. These were known as Pile platforms, after the head of Anti-Aircraft Command, General Frederick Alfred Pile.[9]

In both cases, the saddle rotated 360° on the carriage or pedestal and provided elevation up to 80°. An AEC Matador was the normal gun tractor. There were six marks of ordnance (the barrel and breech assembly) and a few marks of carriage of both versions, some using letter suffixes. The carriage included the recoil system, laying arrangements, fuse setting and loading machinery. The Mk IIC mounting enabled fully automatic engagements, apart from putting shells into the feed to the machine fuze setter.

A 3.7-inch anti-aircraft gun towed by an AEC Matador at Burrow Head, Scotland, early 1944.

Initially, there were HE and shrapnel shells, both fitted with a time fuse. Fuse No.199 was igniferous (i.e. powder-burning) with a maximum running time of 30 seconds. Fuses No. 106 and 107 were mechanical time fuses; both proved unsatisfactory. Fuse No. 208, with a maximum running time of 43 seconds, became the standard fuse. A great improvement in 1942 was the introduction of Machine Fuse Setter No. 11, on Mounting Mk. IIC and Carriage Mk. IIIA, which raised the rate of fire to 20 rounds per minute. The introduction of the VT fuse later in the war further increased the gun's effectiveness, and was particularly useful against the V-1 flying bomb.[10]

Monobloc barrel.

Barrel changed to loose liner.

The Mk III started as a combination of the Mk I breech with the Mk II barrel.

A prototype development of the 3.7-inch gun using the QF 4.5-inch naval gun Mk V barrel with a liner to give a gun using a 4.45 inches (113 mm) size cartridge case to drive the 3.7 inches (94 mm) shell. The barrel wear proved excessive and it was dropped in favour of the Mk VI.

Similar to the Mk IV. Also dropped in favour of the Mk VI.

Like the Mk IV this was based on the 4.5 inch barrel design lined down to 3.7 inches, and using the 4.5 inch size cartridge. However, Colonel Probert changed the barrel to have gradual rifling: the rifling groove depth decreased to zero over the last five calibres of the barrel before the muzzle. This smoothed the two driving bands of a new design shell giving reduced air resistance and hence better ballistic performance, and causing far less barrel wear. The maximum ceiling for the gun was about 15,240 m (50,000 ft). It was mounted on the Mounting Mk IIA and therefore deployed in static emplacements only. In service from 1944 to 1959.

As of 1949, erosive cordite propellant was not used with Mk 6 guns, but instead 17 lb and 2-6 oz of different single-base nitrocellulose propellant (as opposed to slightly over 7 lb 1 oz of cordite or around 8.5-9 lbs of single-base for Marks 1-3 guns).[11]

• Dobinson, Colin (2001). AA command : Britain's anti-aircraft defences of World War II. London: Methuen. ISBN 0-413-76540-7. OCLC 47192791.
• Foss, Christopher F. (1979). Jane's pocket book of towed artillery. New York: Collier Books. ISBN 0-02-080600-0. OCLC 1232304777.
• Frederick, J. B. M. (1984). Lineage book of British land forces 1660-1978. Wakefield: Microform Academic. ISBN 1-85117-009-X. OCLC 18072764.
• Hogg, Ian V. (1998). Allied artillery of World War One. Marlborough: Crowood. ISBN 1-86126-104-7. OCLC 40543410.
• Hogg, Ian V. (1970). The guns: 1939/45. New York: Ballantine Books. ISBN 0-01-906710-0. OCLC 75910.
• Farndale, Martin, Sir; Hughes, B. P. (1986). History of the Royal Regiment of Artillery. Woolwich, London: Royal Artillery Institution. ISBN 1-870114-00-0. OCLC 21298417.{{cite book}}: CS1 maint: multiple names: authors list (link)
• Mandeep Singh (2017). Baptism under fire : anti aircraft artillery in India Pakistan war 1965. New Delhi (India). ISBN 978-93-86457-12-7. OCLC 987797193.{{cite book}}: CS1 maint: location missing publisher (link)

Wikimedia Commons has media related to QF 3.7 inch AA gun.

• 36th Heavy Air Defence Regiment website