QF 3-inch 20 cwt

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Anti-aircraft gun

QF 3 inch 20 cwt anti-aircraft gun
Aboard HMS Royal Oak in World War I
Type	Anti-aircraft gun
Place of origin	United Kingdom
Service history
In service	1914–1947
Used by	United Kingdom Australia Canada Finland Ireland
Wars	World War I World War II
Production history
Designer	Vickers
Variants	Mk I, Mk I, Mk IB, Mk 1C, Mk 1C, Mk IE, Mk SIE, Mk II, Mk III, Mk IV, Mk IVA
Specifications
Mass	2,240 lb (1,020 kg), gun and breech 5.99 tons, with 2-wheel platform
Barrel length	Bore: 11 ft 4 in (3.45 m) (45 cal) Total: 11 ft 9 in (3.58 m)
Crew	11

Shell	Fixed QF HE 76.2 × 420 mm R
Shell weight	12.5 lb (5.7 kg), 1914; 16 lb (7.3 kg), 1916
Calibre	3 inches (76 mm)
Breech	Semi-automatic sliding-block
Recoil	11 inches (280 mm). hydro spring, constant
Carriage	High-angle wheeled, static, or lorry mounting
Elevation	−10 – 90°
Traverse	360°
Rate of fire	16–18 rpm
Muzzle velocity	2,500 ft/s (760 m/s), 12.5 lb (5.7 kg) shell 2,000 ft/s (610 m/s), 16 lb (7.3 kg) shell
Effective firing range	16,000 ft (4,900 m)
Maximum firing range	23,500 ft (7,200 m), 12.5 lb shell 22,000 ft (6,700 m), 16 lb shell

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 (1 cwt = 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.

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.

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; a Welin breech block with an Asbury breech.

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. Routledge quotes a rate of fire of 16–18 rounds per minute, in the context of the 16-pounder shell of 1916. This would appear to be the effective rate of fire found to be sustainable in action.

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. Eight more Mks followed between the World Wars. 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. 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.

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. 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.

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, 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. 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. 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.

By June 1916, 202 3-inch 20 cwt were deployed in the air defence of Britain, of a total of 371 AA guns.

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. 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 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
On HMAS Australia, December 1918

Performance

Comparison with other British World War I anti-aircraft guns
Gun	muzzle velocity	Shell weight	Time to 5,000 feet (1,500 m) at 25° (seconds)	Time to 10,000 feet (3,000 m) at 40° (seconds)	Time to 15,000 feet (4,600 m) at 55° (seconds)	Max. height
QF 13 pdr 9 cwt	1,990 feet per second (610 m/s)	12.5 pounds (5.7 kg)	10.1	15.5	22.1	19,000 feet (5,800 m)
QF 12 pdr 12 cwt	2,200 feet per second (670 m/s)	12.5 pounds (5.7 kg)	9.1	14.1	19.1	20,000 feet (6,100 m)
QF 3-inch 20 cwt 1914	2,500 feet per second (760 m/s)	12.5 pounds (5.7 kg)	8.3	12.6	16.3	23,500 feet (7,200 m)
QF 3-inch 20 cwt 1916	2,000 feet per second (610 m/s)	16 pounds (7.3 kg)	9.2	13.7	18.8	22,000 feet (6,700 m)
QF 4-inch Mk V naval gun	2,350 feet per second (720 m/s)	31 pounds (14 kg)	4.4??	9.6	12.3	28,750 feet (8,760 m)

World War II

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. 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 However, it was discovered at mobilization that the 233 guns in HAA reserve were missing various parts and predicted fire instruments. 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.

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. 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)".

Naval gun

In World War II the gun was carried by S-class, U-class and V-class submarines.

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.

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
Churchill gun carrier in Dorset, March 1943
On a U-class submarine, April 1943

Finnish use

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.

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

Surviving examples

At the Royal Artillery Museum, London
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.

References

^ Hogg & Thurston 1972, page 78
^ Campbell, Naval Weapons of WWII, p.61-62.
^ Hogg & Thurston 1972, page 79
^ Farndale 1988, page 397
Routledge 1994, page 9, 13
"77-77 MM CALIBRE CARTRIDGES". www.quarryhs.co.uk. Archived from the original on 17 January 2015. Retrieved 12 September 2017.
^ Routledge 1994, page 12
^ Routledge 1994, page 13
^ Routledge 1994, page 9
Navweaps.com
Notes on Anti-Aircraft Guns. April 1917, page 22
Routledge 1994, page 43
Postan, Michael Moïssey; Hay, Denys; Scott, John Dick (1964). Design and Development of Weapons. Studies in Government and Industrial Organisation. H.M. Stationery Office. p. 284. ISBN 978-0-11-630089-8.
Routledge 1994, page 50
Routledge 1994, page 50-51
Routledge 1994, page 4-5
Rawlinson, Alfred, Sir, The defence of London, 1915–1918, 1923 (p.57)
Routledge 1994, page 24
Hogg & Thurston 1972, page 220
Hogg & Thurston 1972, page 68
Farndale 1986, page 364
Routledge 1994, page 27. Farndale 1988, page 342 quotes 56 in service in France (meaning Western Front) at the Armistice.
Routledge 1994, Page 9
Hogg & Thurston 1972, Page 234-235
Routledge 1994, Page 13
"RAAHC - Artillery Register - Manly, NSW, QF 3 inch 20 CWT Anti Aircraft Gun, Serial Number 4282".
Routledge 1994, page 50.
Routledge 1994, page 371
Routledge 1994, page 125
Nigel F Evans,British Artillery In World War 2. Anti-Tank Artillery
Jaeger Platoon: Finnish Army 1918 – 1945 Antiaircraft Guns Part 3: Heavy Guns
"Fortress Dover | English Heritage". www.english-heritage.org.uk. Retrieved 11 January 2017.

Bibliography

Farndale, Sir Martin (1986). Western Front 1914–18. History of the Royal Regiment of Artillery. London: Royal Artillery Institution. ISBN 1-870114-00-0..
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. ISBN 1-870114-05-1.
Hogg, I. V.; Thurston, L.F. (1972). British Artillery Weapons & Ammunition 1914–1918. London: Ian Allan. ISBN 978-0-7110-0381-1.
Routledge, Brigadier NW (1994). Anti-Aircraft Artillery, 1914–55. History of the Royal Regiment of Artillery. London: Brassey's. ISBN 1-85753-099-3.
Notes on Anti-Aircraft Guns, US Army War College, April 1917 – via Combined Arms Research Library
Campbell, John (1985). Naval Weapons of World War Two. Naval Institute Press. ISBN 978-0-87021-459-2.

External links

v t e British Empire naval weapons of the First World War
Monitor guns	BL 9.2-inch Mk VI BL 9.2-inch Mk VIII BL 9.2-inch Mk X BL 9.2-inch Mk XII BL 12-inch Mk VIII BL 14-inch Mk II (US built) BL 15-inch Mk I BL 18-inch Mk I
Capital ship main armament	Vickers 10-inch 45 cal. EOC 10-inch 45 cal. BL 12-inch Mk VIII BL 12-inch Mk IX BL 12-inch Mk X BL 12-inch Mk XI & XII EOC 12-inch 45 cal. Mk XIII BL 13.5-inch Mk I – IV BL 13.5-inch Mk V BL 13.5-inch Mk VI EOC 14-inch 45 cal. Mk I BL 15-inch Mk I
Armoured cruiser main armament	BL 7.5-inch Mk I BL 9.2-inch Mk X BL 9.2-inch Mk XI
Secondary armament and light cruiser main armament	QF 12-pounder 18 cwt QF 14-pounder Maxim-Nordenfelt QF 14-pounder Mk I & II QF 4-inch Mk III BL 4-inch Mk VII QF 4-inch Mk V BL 4-inch Mk IX BL 5.5-inch Mk I QF 6-inch Mk I – III BL 6-inch Mk VII BL 6-inch Mk XI BL 6-inch Mk XII BL 6-inch Mk XIII-XVIII BL 7.5-inch Mk II – Mk V
Destroyer and small cruiser armament	QF 12-pounder 12 cwt Mk II QF 4-inch Mk III BL 4-inch Mk VIII QF 4-inch Mk IV BL 4.7-inch Mk I
Merchant ship armament	QF 4.7-inch Mk I – IV QF 4.7-inch Mk V*
Submarine guns	BL 4-inch Mk XI QF 4-inch Mk XII
Anti-aircraft guns	QF 2-pounder "pom-pom" Mk II QF 3-inch 20 cwt QF 4-inch Mk V
Light weapons	.303 Vickers machine gun QF 3-pounder Hotchkiss QF 3-pounder Vickers QF 6-pounder Hotchkiss
Torpedoes	18-inch Mk V – VIII 21-inch Mk II 21-inch Mk IV
Mines	Spherical Mk I, II and III Vickers Elia mine Type H mine
Anti-submarine weapons	Type A Depth charge Type B Depth charge Type C Depth charge Type D Depth charge Type E Depth charge Type F Depth charge Type G Depth charge Cruiser Mine 7.5-inch howitzer
Landing guns	QF 12-pounder 8 cwt

v t e British Empire artillery of the First World War
Armoured vehicle guns	QF 3-pounder Vickers QF 6-pounder Hotchkiss QF 6-pounder 6 cwt
Infantry guns	1.59-inch breech-loading Vickers Q.F. gun, Mk II
Field artillery	BL 12-pounder 6 cwt QF 12-pounder 8 cwt QF 12-pounder 18 cwt QF 13-pounder BL 15-pounder BLC 15-pounder QF 15-pounder QF 18-pounder QF 4-inch gun Mk III BL 4-inch gun Mk VII QF 4.5-inch howitzer
Medium & heavy artillery	QF 4.7-inch gun BL 5-inch howitzer BL 5.4-inch howitzer BL 60-pounder gun BLC 6-inch siege gun BL 6-inch gun Mk VII BL 6-inch gun Mk XIX BL 6-inch 26 cwt howitzer BL 6-inch 30 cwt howitzer BL 8-inch howitzer Mk I – V BL 8-inch howitzer Mk VI – VIII
Siege artillery	BL 7.5-inch Mk III naval gun BL 9.2-inch howitzer BL 9.2-inch Mk X naval gun BL 12-inch howitzer BL 12-inch Mk X naval gun BL 15-inch howitzer
Mountain artillery	RML 2.5-inch mountain gun BL 10-pounder mountain gun BL 2.75-inch mountain gun QF 2.95-inch mountain gun QF 3.7-inch mountain howitzer
Mortars	Garland trench mortar 3-inch Stokes mortar Vickers 1.57-inch mortar 2-inch medium mortar Newton 6-inch mortar 9.45-inch Heavy Mortar
Smoke and chemical weapons	4-inch Stokes Mortar Livens Projector Livens Large Gallery Flame Projector
Anti-aircraft guns	QF 1-pounder pom-pom QF 2-pounder "pom-pom" Mk II 75 mm AA gun QF 12-pounder 12 cwt QF 13-pounder 6 cwt QF 13-pounder Mk IV QF 13-pounder 9 cwt QF 3-inch 20 cwt QF 18-pounder QF 4-inch Mk V
Coastal artillery	QF 12-pounder 12 cwt QF 4-inch naval gun Mk I – III BL 6-inch Mk VII naval gun BL 9.2-inch Mk IX – X naval gun RML 9-inch
Railway guns	BL 9.2-inch railway gun BL 12-inch railway gun BL 12-inch railway howitzer BL 14-inch railway gun

v t e British Commonwealth artillery of World War II
Tank guns	QF 2-pounder QF 3-pounder QF 6-pounder QF 75 mm QF 3-inch howitzer QF 17-pounder 77 mm HV QF 25-pounder Short QF 95 mm howitzer QF 32-pounder
Anti-tank guns	QF 2-pounder QF 6-pounder QF 17-pounder QF 3-inch 16 cwt
Field guns and howitzers	75 mm Gun M1917 QF 18-pounder 25-pounder Gun-Howitzer 25-pounder Short QF 4.5-inch howitzer
Medium and heavy guns and howitzers	BL 4.5-inch medium field gun BL 60-pounder gun BL 5.5-inch medium gun BL 6-inch howitzer BL 6-inch gun Mk XIX 155 mm Long Tom BL 7.2-inch howitzer BL 8-inch howitzer BL 9.2-inch howitzer 240 mm howitzer M1
Mountain guns	75mm Pack howitzer M1 3.7-inch mountain howitzer
Mortars	SBML 2-inch mortar ML 3-inch mortar ML 4.2-inch mortar
Anti-aircraft weapons	Z Battery 20 mm Oerlikon 20 mm Polsten QF 1½-pounder Mk III QF 2-pounder naval gun Bofors 40 mm QF 3-inch QF 3.7-inch QF 4.5-inch Mk II QF 5.25-inch Mk II
Coast defence	QF 6-pounder 10 cwt QF 12-pounder QF 4.7-inch Mk I–IV BL 6-inch Mk VII & Mk XXIV BL 7.5-inch Mk VI BL 8-inch Mk VIII BL 9.2-inch Mk X BL 14-inch Mk VII BL 15-inch Mk I
Railway artillery	BL 9.2-inch Mk XIII railway gun BL 12-inch Mk V railway howitzer BL 13.5-inch Mk V railway gun BL 18-inch railway howitzer

v t e British Commonwealth naval weapons of the Second World War
Monitor guns	BL 15-inch
Capital ship main armament	BL 14-inch BL 15-inch BL 16-inch
Heavy cruiser main armament	BL 7.5-inch Mk VI BL 8-inch Mk VIII
Secondary armament and light cruiser main armament	QF 4-inch Mk V QF 4.5-inch QF 5.25-inch BL 5.5-inch BL 6-inch Mk XII BL 6-inch Mk XXII BL 6-inch Mk XXIII
Minesweeper, Sloop, Corvette, Frigate and Destroyer armament	QF 6-pounder 10 cwt QF 12-pounder 12 cwt Mk V QF 4-inch Mk IV BL 4-inch Mk IX QF 4-inch Mk XVI QF 4-inch Mk XIX QF 4.5-inch BL 4.7-inch Mk II QF 4.7-inch Mk IX & XII QF 4.7-inch Mk XI
Submarine guns	QF 12-pounder 12 cwt Mk V QF 3-inch 20 cwt QF 4-inch Mk XII & XXII
Anti-aircraft weapons	Unrotated projectile Holman Projector 20 mm Oerlikon QF 2-pounder "pom-pom" Mk VIII 40 mm Bofors QF 3-inch 20 cwt QF 4-inch Mk V QF 4-inch Mk XVI QF 4.5-inch QF 4.7-inch Mk VIII
Light weapons	.303 Lewis machine gun .50 Vickers Mk III machine gun QF 3-pounder Vickers
Torpedoes	21-inch Mk VII 21-inch Mk VIII 21-inch Mk X 21-inch Mk XI 24.5-inch Mk I
Anti-submarine weapons	Thornycroft depth charge thrower Hedgehog Squid
Small craft armament	Molins 6-pounder Vickers K machine gun 18-inch torpedo
Foreign-sourced weapons	20 mm Oerlikon 40 mm Bofors 3-inch 23 caliber gun (US) 3-inch 50 caliber gun (US) 4-inch 50 caliber Mark 9 gun (US) 5-inch 51 caliber Mark 8 gun (US) Y-gun depth charge projector (US) Mark 8 torpedo (US)

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