- QF 3 inch 20 cwt
name=QF 3 inch 20 cwt anti-aircraft gun
caption=Egyptian gun captured by Israel in the 1956 war.
service=1914 - 1946Hogg & Thurston 1972, page 78]
World War I World War II
weight=Gun & breech : convert|2250|lb|abbr=on
Total on 2 wheel platform : 5.99 tons
part_length=Bore 135 inch (45 cal)
crew=11 [Farndale 1988, page 397]
caliber=3 inch (76 mm)
12.5 lb (1914);
16 lb HE (1916) [Routledge 1994, page 9, 13]
rate=16-18 rpmRoutledge 1994, page 13]
velocity=2500 ft/s (12.5 lb shell)
2000 ft/s (16 lb shell)Routledge 1994, page 9]
range=16,000 ftRoutledge 1994, page 13]
max_range=22,000 ft (16 lb shell)Routledge 1994, page 13]
23,500 ft (12.5 lb shell)
breech=semi-automatic sliding blockRoutledge 1994, page 12]
recoil=11 inches. Hydro spring, constantHogg & Thurston 1972, page 79]
carriage=high-angle wheeled, static or lorry mounting
elevation=-10° - 90°Hogg & Thurston 1972, page 79]
traverse=360 °The QF 3 inch 20 cwt anti-aircraft gun became the standard anti-aircraft gun used in the home defence of England against German airships and bombers in
World War I. It was also common on British warships in World War Iand 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, hence the barrel and breech together weighed 2250 lb). While small numbers of other "3 inch" AA guns were used, the 3 inch 20 cwt gun is what writers are usually referring to by "3 inch AA gun".
Design and development
The gun was based on a prewar
Vickersnaval convert|3|in|mm|sing=on QF gun with modifications specified by the War Officein 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 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.Routledge 1994, page 12]
The early convert|12.5|lb|abbr=on shrapnel shell at convert|2500|ft/s|m/s|abbr=on caused excessive barrel wear and was unstable in flight. The 1916 16 lb shell at convert|2000|ft/s|m/s|abbr=on proved ballistically superior and was better suited to a high explosive filling.Routledge 1994, page 13]
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 blockwith an Asbury breech. [http://www.navweaps.com/Weapons/WNBR_3-45_mk1.htm Naval weapons]
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. [Notes on Anti-Aircraft Guns. April 1917, page 22] 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 was introduced to replace the obsolete lorries still used as mounts from World War I, together with modern new barrels, and equipment to connect the guns to the new Vickers No. 1 Predictor. [Routledge 1994, page 43] 8 more Mks followed between the World Wars. From 1934 the rocking-bar deflection sights were replaced by Magslip receiver dials which received input from the Predictor.Routledge 1994, page 50] Predictor No. 1 was superseded from 1937 by Predictor No. 2, based on a US Sperry AAA Computer M3A3. This was faster and could track targets at convert|400|mi/h|km/h|abbr=on at heights of convert|25000|ft|m|abbr=on [Routledge 1994, page 50-51]
The 3 inch 20 cwt gun was superseded by the
QF 3.7 inch AA gunfrom 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 Mk V on HA (high-angle) mounting.
World War I
On HMAS Australia, December 1918
World War Iwith no anti-aircraft artillery. When war broke out and Germany occupied Belgium and North-east France, it was realised that key installations at home could be attacked by air, the government began a search for suitable anti-aircraft guns. The Navy provided the initial convert|3|in|mm|sing=on 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. [Routledge 1994, page 4-5] It was from then onwards operated by Royal Garrison Artillerycrews, with drivers and crew for motor lorries provided by the Army Service Corps.
Other earlier anti-aircraft guns based on the existing 13 pounder and 18 pounder guns proved inadequate, apart from the
QF 13 pounder 9 cwtbut 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 [Routledge 1994, page 24] convert|16|lb|abbr=on shell and fairly high altitude was well suited to defending England against high-altitude Zeppelins and bombers. The 16 pound shell took 9.2 seconds to reach convert|5000|ft|m|abbr=on at 25° from horizontal, 13.7 seconds to reach convert|10000|ft|m|abbr=on at 40°, 18.8 seconds to reach 15,000 at 55°.Routledge 1994, page 9] This means that the gun team had to calculate where the target would be 9 - 18 seconds ahead, choose the correct fuze setting, load, aim and fire accordingly.
Previous British time-delay fuzes developed for land use had been discovered to be unsuitable for the new vertical shooting, as the gunpowder used in the delay mechanism burned differently and unpredictably at altitude. The new fuze had a modified mechanism and special powder formulation to provide predictable burn speed and hence delay at altitude. Britain lagged behind Germany in switching to clockwork time-delay fuzes. In addition, experience showed that the traditional mechanism in previous time fuzes which detonated the shell on impact even if the time delay failed, had to be disabled, as unexploded AA shells would land amongst those firing them and nearby civilians and hence must not explode on landing. [Hogg & Thurston 1972, page 220]
The carriage's short recoil of convert|11|in|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. [Hogg & Thurston 1972, page 68]
By June 1916, 202 3 inch 20 cwt were deployed in the air defence of Britain, of a total of 371 AA guns. [Farndale 1988, page 397]
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. [Farndale 1986, page 364] An AA section consisted of 2 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 [Routledge 1994, page 27. Farndale 1988, page 342 quotes 56 in service in France (meaning Western Front) at the Armistice.] mounted on heavy lorries, typically the Peerless 4 Ton. In addition, many were mounted on Royal Navy ships.
The following tableRoutledge 1994, Page 9] compares the gun's performance with the other British WWI anti-aircraft guns:-
World War II
At the beginning of
World War IIin 1939, Britain possessed approximately 500 of these guns, some deployed as light anti-aircraft guns (LAA) for airfield defence until more 40mm Bofors guns arrived, others as heavy anti-aircraft (HAA) guns until numbers of the new 3.7 inch gun arrived.Routledge 1994, page 50.] However, it was discovered at mobilization that the 233 guns in HAA reserve were missing various parts and predicted fire instruments. [Routledge 1994, page 371] 120 were in France with the British Expeditionary Forcein November 1939, compared with 48 of the modern QF 3.7 inch AA gun. [Routledge 1994, page 125]
In 1941, 100 of the by-now obsolete guns were converted to become the 3 inch 16 cwt anti-tank gun, firing a convert|12.5|lb|abbr=on armour-piercing shell. [Nigel F Evans, [http://members.tripod.com/~nigelef/anti-tank.htm BRITISH ARTILLERY IN WORLD WAR 2. ANTI-TANK ARTILLERY] ] They appear to have been mainly deployed in home defence.
Britain supplied 24 Mk 3 guns and 7 M/34 mechanical fire control computers to Finland during the
Winter Warof 30 November 1939- March 1940 but they arrived too late to be used. They were used during the Continuation Warof 1941 - 1944. [ [http://www.jaegerplatoon.net/AA_GUNS3.htm#76ItK16 Jaeger Platoon: Finnish Army 1918 - 1945 Antiaircraft Guns Part 3: Heavy Guns] ]
World War I ammunition
3-inch M1918 gun
List of artillery#Anti-aircraft guns
*At the [http://www.firepower.org.uk Royal Artillery Museum, London]
*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. See for photographs.
*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. See for photographs.
*Mk 3 gun is displayed at the [http://www.ilmatorjuntamuseo.fi/eng Ilmatorjuntamuseo, Tuusula Finland]
*General Sir Martin Farndale, [http://www.naval-military-press.com/FMPro?-db=nmp%5fproducts.fp5&-format=nmpweb%2fdetail.htm&-lay=cgi&-sortfield=date&Co=NMP&search=History%20of%20the%20Royal%20Regiment%20of%20Artillery.%20Western%20Front%201914-18&-max=20&-recid=35933&-token.0=3668698&-find= History of the Royal Regiment of Artillery. Western Front 1914-18. London: Royal Artillery Institution, 1986]
*General Sir Martin Farndale, [http://www.amazon.co.uk/dp/1870114051 History of the Royal Regiment of Artillery : Forgotten Fronts and the Home Base 1914-18. London:The Royal Artillery Institution, 1988]
*I.V. Hogg & L.F. Thurston, British Artillery Weapons & Ammunition 1914-1918. London: Ian Allan, 1972.
*Brigadier NW Routledge, [http://www.amazon.co.uk/History-Royal-Regiment-Artillery-Anti-aircraft/dp/1857530993 History of the Royal Regiment of Artillery. Anti-Aircraft Artillery, 1914-55. London: Brassey's, 1994]
* [http://cgsc.cdmhost.com/u?/p4013coll9,222 Notes on Anti-Aircraft Guns. US Army War College, April 1917. Includes report on 3 inch 20 cwt.] Provided online by Combined Arms Research Library
* [http://www.navweaps.com/Weapons/WNBR_3-45_mk1.htm British 12-pdr (3"/45 (76.2 cm)) 20cwt QF HA Marks I, II, III and IV]
*Ken Musgrave, [http://www.landships.freeservers.com/new_pages/musgraves_plans.htm Diagram of 3 inch 20 cwt gun on Peerless 4 Ton Lorry]
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