QF 3.7 inch AA gun

QF 3.7-in heavy anti-aircraft gun
A 3.7-inch gun on a travelling carriage in London in 1939
Type	Anti-aircraft gun
Place of origin	United Kingdom
Service history
In service	1937–present (the Nepalese Army still has 45 in service)
Used by	UK and Commonwealth
Wars	World War II
Production history
Designer	Vickers
Designed	1937
Produced	1937–1945
No. built	approx. 10.000
Specifications
Weight	20,541 pounds (9,317 kg)
Length	4.96 m (16 ft 3 in)
Barrel length	4.7 m (15 ft 5 in) L/50
Crew	7
Shell	28 pounds (13 kg)
Calibre	3.7 inches (94 mm)
Carriage	Mobile and static versions
Elevation	−5 to +80 degrees
Traverse	360 degrees
Rate of fire	10–20 rpm
Muzzle velocity	Mk I–III: 2,670 ft/s (810 m/s) (new) 2,598 ft/s (792 m/s) (worn) Mk VI : 3,425 ft/s (1,044 m/s)
Maximum firing range	Maximum horizontal: 18,800 feet (5,700 m) maximum slant: 12,000 metres (39,000 ft) ceiling Mk I–II: 9,000 metres (30,000 ft) Mk VI: 13,716 metres (45,000 ft)

The QF 3.7-inch AA was Britain's primary heavy anti-aircraft gun during World War II. It was roughly the equivalent of the German 88 mm FlaK and American 90 mm, but with a slightly larger calibre of 94 mm. It was used throughout World War II in all theatres except the Eastern Front. The gun was produced in six major variants, two versions (mobile and fixed) and in considerable numbers. The Mk VI ordnance used only with a fixed mounting gave vastly increased performance. It remained in use after the war until AA guns were replaced by guided missiles, notably the Thunderbird, in the late 1950s.

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:

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 fuzes were required because the speed of powder burning varied with height so fuze 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.