23,500 ft (7,200 m), 12.5lb shell[3] 22,000 ft (6,700 m), 16lb shell[8]
The QF 3-inch 20 cwt anti-aircraft gun became the standard anti-aircraft gun used in the home defence of the United Kingdom against German
Zeppelins airships and bombers and on the Western Front in
World War I. It was also common on British warships in World War I and submarines in
World War II. 20
cwt referred to the weight of the barrel and breech, to differentiate it from other 3-inch guns (1cwt = 1
hundredweight = 112 lb, 51 kg, hence the barrel and breech together weighed 2,240 lb, 1,020 kg). While other AA guns also had a bore of 3 inches (76 mm), the term 3-inch was only ever used to identify this gun in the World War I era, and hence this is what writers are usually referring to by 3-inch AA gun.
Design and development
The gun was based on a prewar
Vickers naval 3-inch (76 mm) QF gun with modifications specified by the
War Office in 1914. These (Mk I) included the introduction of a vertical
sliding breech-block to allow semi-automatic operation. When the gun recoiled and ran forward after firing, the motion also opened the breech, ejected the empty cartridge case and held the breech open ready to reload, with the striker cocked. When the gunner loaded the next round, the block closed and the gun fired.[7]
The early 12.5-pound (5.7 kg) shrapnel shell at 2,500 ft/s (760 m/s) caused excessive barrel wear and was unstable in flight. The 1916 16-pound (7.3 kg) shell at 2,000 ft/s (610 m/s) proved ballistically superior and was better suited to a high explosive filling.[8]
The Mark I* had different rifling. The Mark II lost the semi-automatic action.
The Mk III of 1916 reverted to a 2-motion screw breech to suit available manufacturing capability, and Mk IV had a single-tube barrel and single-motion screw breech;[1] a
Welin breech block with an Asbury breech.[10]
A US Army report on anti-aircraft guns of April 1917 reported that this gun's semi-automatic loading system was discontinued because of difficulties of operation at higher angles of elevation, and replaced by "the standard Vickers-type straight-pull breech mechanism", reducing rate of fire from 22 to 20 rds/minute.[11] Routledge quotes a rate of fire of 16–18 rounds per minute, in the context of the 16-pounder shell of 1916.[8] This would appear to be the effective rate of fire found to be sustainable in action.
Guns on 4-wheeled trailers with Vickers Predictor at left, Australia c. 1937
Beginning in 1930, a new towed 4-wheeled sprung trailer platform on pneumatic tyres was introduced to replace the obsolete lorries and two-wheeled carriages still used as mounts from World War I, barrels were equipped with loose liners, and the guns were connected to the new Vickers No. 1 Predictor.[12][13]
Eight more Mks followed between the World Wars.[1] By 1934 the rocking-bar deflection sights had been replaced by Magslip receiver dials which received input from the Predictor, with the layers matching pointers instead of tracking the target.[14] Predictor No. 1 was supplemented from 1937 by Predictor No. 2, based on a US Sperry AAA Computer M3A3. This was faster and could track targets at 400 mph (640 km/h) at heights of 25,000 ft (7,600 m), both Predictors received height data, generally from the Barr & Stroud UB 7 (9-foot base) instrument.[15]
The 3-inch 20 cwt gun was superseded by the
QF 3.7-inch (94 mm) AA gun from 1938 onwards, but numbers of various Marks remained in service throughout
World War II. In Naval use it was being replaced in the 1920s by the
QF 4-inch (100 mm) Mk V on HA (high-angle) mounting.
Combat use
World War I
Britain entered
World War I with no anti-aircraft artillery. When war broke out and Germany occupied Belgium and North-east France, it was realised that key installations in England could be attacked by air. As a result, a search for suitable anti-aircraft guns began. The Navy provided the initial 3-inch (76 mm) guns from its warships, approximately 18 by December 1914, for the defence of key installations in Britain, manned by
RNVR crews, until the new specialised anti-aircraft version began production and entered service.[16] It was from then onwards operated by
Royal Garrison Artillery crews, with drivers and crew for motor lorries provided by the
Army Service Corps. However, the Mobile Anti-Aircraft Brigade based at
Kenwood Barracks in London, continued to be manned by the RNVR, although under the operational control of the Army.[17]
Other earlier anti-aircraft guns based on the existing
13-pounder and
18-pounder guns proved inadequate, apart from the
QF 13-pounder 9 cwt but even that could not reach high altitudes and fired a fairly light shell. The 3-inch 20 cwt with its powerful and stable in flight,[18] 16 lb (7.3 kg) shell and fairly high altitude was well suited to defending the United Kingdom against high-altitude
Zeppelins and bombers. The 16-pound shell took 9.2 seconds to reach 5,000 ft (1,500 m) at 25 degrees from horizontal, 13.7 seconds to reach 10,000 ft (3,000 m) at 40 degrees, 18.8 seconds to reach 15,000 ft (4,600 m) at 55 degrees.[9] This means that the gun team had to calculate where the target would be 9 to 18 seconds ahead, determine the deflection and set the correct
fuze length, load, aim and fire accordingly. Deflection was calculated mechanically and graphically using an optical height & rangefinder to provide data for the two piece Wilson-Dalby 'predictor', with the fuze length read off a scale mounted on the gun.
British time fuzes, required for airburst shooting, were powder burning (igniferous). However, the powder burning rate changed as air pressure reduced, making them erratic for the new vertical shooting. Modified fuzes reduced the variability but did not cure the problem. Britain lagged behind Germany in developing clockwork time fuzes. In addition, experience showed that the percussion mechanism in time fuzes, which burst the shrapnel shell on impact if the timer failed, had to be removed because AA shells could land among friendly troops and nearby civilians.[19] Igniferous fuzes had to have a
gaine in order to detonate HE shells.
The carriage's short recoil of 11 inches (280 mm) allowed a higher rate of fire than for AA guns based on long-recoil field guns such as the
QF 13-pounder 9 cwt.[20]
By June 1916, 202 3-inch 20 cwt were deployed in the air defence of Britain, of a total of 371 AA guns.[4]
The first guns arrived on the
Western Front in November 1916 and by the end of 1916 it equipped 10 sections out of a total of 91.[21] An AA section consisted of two guns and became the standard organizational unit.
By the end of World War I, 257 (out of a total of 402 AA guns) were in land service in England on static and lorry mountings, and 102 (out of a total of 348) were in service on the Western Front[22] mounted on heavy lorries, typically the Peerless 4 Ton. In addition, many were mounted on Royal Navy ships.
Demonstration of towing on 2-wheeled travelling platform
Demonstration of deployment for action on cruciform travelling platform with wheels removed
Mk I gun on Mk IV mounting on Peerless 4 ton lorry, WWI
At the beginning of
World War II in 1939, Britain possessed approximately 500 of these guns. Over the 1930s, they had been gradually modernized, and a limited production line was set up at the Commonwealth Ordnance Factory
Maribyrnong, Australia.[26] Initially most were in the heavy anti-aircraft (HAA) role until replaced by the new
QF 3.7-inch (94 mm) gun. Some were deployed as light anti-aircraft guns (LAA) for airfield defence, being transferred to the
RAF Regiment when this was formed in 1942, until more
40 mm Bofors guns arrived[27] However, it was discovered at mobilization that the 233 guns in HAA reserve were missing various parts and predicted fire instruments.[28] One hundred and twenty were in France with the
British Expeditionary Force in November 1939, compared with 48 of the modern
QF 3.7-inch AA gun.[29]
In 1941, 100 of the obsolete guns were converted to become the 3-inch 16 cwt anti-tank gun, firing a 12.5-pound (5.7 kg) armour-piercing shell.[30] They appear to have been mainly deployed in home defence. Some were mounted to
Churchill tanks to become the "Gun Carrier, 3-inch, Mk I, Churchill (A22D)".
It was also fitted to older destroyers,
A-class to
I class during refits in 1940, replacing a set of torpedo tubes, to increase their AA capabilities. Some smaller warships used this gun as well. In 1939 it was estimated the RN had 553 Mk I, 184 Mk II, 27 Mk III and 111 Mk IV guns in service.[2]
A modernized 3-incher of
6th HAA Bde towed by an AEC truck, 1934
An Australian gun of the same type with a Vickers predictor in
Narrabeen, 1937
A static mount of the 99th Anti-Aircraft Regiment in Kent, May 1940
Britain supplied 24 Mk 3 guns and 7 M/34 mechanical fire control computers to Finland during the
Winter War of 30 November 1939 to March 1940 but they arrived too late to be used. They were used during the
Continuation War of 1941–1944.[31]
World War I ammunition
Mk I 12.5 lb (5.7 kg) shrapnel round 1914
Mk I 12.5 lb (5.7 kg) shrapnel shell 1914
No. 84 Time & Percussion Fuze for shrapnel shells
No. 80/44 Time Fuze with No. 2 Gaine (left) and 44/80 Gaine (right) for high-explosive shells
World War II ammunition
Royal Navy gunner with fixed round in World War II
A gun at
Pendennis Castle, Cornwall, UK, now displayed at Dover Castle, Kent, UK
A gun captured by Israel from Egypt in the 1956 war, missing breech screw, is displayed at the Clandestine Immigration and Naval Museum, Haifa, Israel.
A gun from the Egyptian ship "El Amir Faruk", sunk in 1948, missing the elevation mechanism, is displayed at the Clandestine Immigration and Naval Museum, Haifa, Israel.
Mk 3 gun is displayed at the Ilmatorjuntamuseo, Tuusula, Finland.
Mk III at Oulu garrison,
Intiö district of Oulu, Finland
A restored gun at
Dover Castle, Kent, UK. During the summer months, weekly demonstration firings take place.[32]
Farndale, Sir Martin (1986). Western Front 1914–18. History of the Royal Regiment of Artillery. London: Royal Artillery Institution.
ISBN1-870114-00-0..
Farndale, General Sir Martin (1988). Forgotten Fronts and the Home Base 1914–18. History of the Royal Regiment of Artillery. London: The Royal Artillery Institution.
ISBN1-870114-05-1.