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QF 3.75 inch AA: Wikis


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QF 3.7-in Heavy Anti-aircraft Gun
3,7in AA Gun-Aberdeen.00051hw5.jpg
3.7 inch Anti-Aircraft Gun on display at the U.S. Army Ordnance Museum in Aberdeen, Maryland
Type Anti-aircraft gun
Place of origin  United Kingdom
Service history
In service 1937–1959
Used by UK and Commonwealth
Wars World War II
Production history
Designed 1937
Produced 1937–
Weight 20,541 lb (9,317 kg)
Length 4.96 m
Barrel length /L50 185 inches (4.7 m)
Crew 7

Shell 28 lb (12.7 kg)
Calibre 3.7 inches (94 mm)
Carriage Mobile and static versions
Rate of fire 10/20 rpm
Muzzle velocity 792 m/s (2,598 ft/s)
Maximum range Maximum horizontal 18,800 m (61,679 ft)
Maximum slant 12,000 m (39,370 ft)
Ceiling 9,000 m (29,527 ft)

The 3.7-Inch QF AA was Britain's primary heavy anti-aircraft gun during World War II, the equivalent of the German 88 mm FlaK with a slightly larger calibre of 94 mm. It remained in use after the war until AA guns were replaced by guided missiles in the late 1950s.



Prior to World War II most countries had not seriously considered upgrading their anti-aircraft systems. The rapid advances in aircraft performance in the early 1930s due to the widespread introduction of supercharging, notably a dramatic increase in operating altitude, meant that guns had marginal performance against newer aircraft. Adding to the concern was the much higher speeds of aircraft, which reduced detection times and seemed to make engagements very difficult to arrange. Given these problems, the British relied almost entirely on the Royal Air Force's bomber fleet as a deterrent force, ignoring the defence almost entirely.

Attitudes started to change in the mid-1930's with the introduction of radar. This so improved the detection time and tracking capabilities that defence once again seemed like a real possibility. New fighter designs like the Supermarine Spitfire overtook the bombers in the production queues in order to build up a real defence. Little thought had been put into new AAA systems, and their existing World War I-era QF 3 inch AA guns lacked the capability to reach the altitudes of the newer aircraft. A new gun was required that would have the performance to deal with targets flying as high as 30,000 ft, which led to the development of the 3.7 inch.

Development was fairly rapid, and production examples entered into service as early as 1937. 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 (76 mm) guns. This number increased to 341 by September 1938 (Munich Crisis), to 540 in September 1939 (declaration of war), and to 1,140 during the Battle of Britain. Production continued through a variety of improved versions, leading eventually to the Mark VI that remained in service until 1959. It was used on a static mount, or a mobile mount with a cruciform set of legs for stability in the deployed position. To move, the legs were folded in and a pair of road wheels lowered, the gun would then be hitched to an AEC Matador gun tractor.

3.7 inch gun in London in 1939.

Anti-tank capability

The 3.7-inch (94 mm) gun was never used as an anti-tank weapon, except in one or two emergencies. This is in contrast to the German Army, which integrated their equivalent "88" into anti-tank defensive screens from 1940 onwards.

This was mainly because the 3.7-inch (94 mm) gun mobile mounting was almost twice as heavy as the German "88". Redeploying it was a slower operation, and the heavy AEC Matador artillery tractor normally used for towing could operate on hard surfaces only. Additionally, heavy AA Regiments equipped with the 3.7-inch (94 mm) gun were controlled by Corps or Army HQ, or at even higher level HQs, and command of them was not often devolved to the commanders at Divisional levels where the anti-tank role might be required. Prolonged firing at low elevations (not part of the original specification) also strained the mounting and recuperating gear.

The gun was used as the basis for the Tortoise assault tank's 32-pounder anti-tank gun, but this tank, which is best described as a self-propelled gun, never saw service.


A Nr.1 Mark III Predictor that was used with the QF 3.7

Mk I

The original version featured an "advanced" recoil system with oil buffer and a compressed air recuperator. The recuperator both helped to absorb the recoil and also returned the piece to the firing position. In field conditions, the compressed air system leaked slightly, and needed frequent topping up from a compressed air cylinder.


Slightly different from the Mk I in build up of breech and barrel. Manufactured in the UK by Vickers-Armstrong until 1943 when production was taken on by Canada as the 3.7-Inch AA Mark II C.


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


The Mk IIIA was similar to the Mk III in terms of the gun, but changed almost all of the ancillary equipment and was a major improvement over the earlier versions. Of particular note, the Mk IIIA introduced an automatic fuze-setter and an automatic loader. This both improved the rate of fire and eliminated the unavoidable variations caused by manual loading and fuze setting, leading to consistent firing which made better use of the predictor data.


A prototype development of the 3.7 by using the Naval QF 4.5-inch (110 mm) naval gun Mk V with a liner to give a gun using a 4.5-inch (110 mm) size shell cartridge case to drive the 3.7-inch (94 mm) shell. Dropped in favour of the Mk VI.

Mk V

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

Mark VI

Using a Naval 5.25-inch (133 mm) mount and a longer 3.7-inch (94 mm) barrel as a starting point, at the Royal Arsenal, Woolwich, Colonel Probert developed a barrel with graduated rifling: the rifling depth decreased to zero over the last five calibres of the barrel before the muzzle. This smoothed the driving bands of the shell flush, giving it a more aerodynamic shape and hence better ballistic performance. The ceiling for the gun was about 15,240 metres (50,000 ft). It was mounted on a 4.5-inch (110 mm) carriage, which was too heavy for a towed version to be economically viable, so the Mk VI was deployed in static emplacements only. In service from 1944 to 1959.


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