| QF 3 inch 20 cwt anti-aircraft gun | |
|---|---|
 AboardHMS Royal Oak in World War I | |
| Type | Anti-aircraft gun |
| Place of origin | United Kingdom |
| Service history | |
| In service | 1914–1947[1] |
| 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[2] |
| Specifications | |
| Mass | 2,240 lb (1,020 kg), gun and breech 5.99 tons,[3] with 2-wheel platform |
| Barrel length | Bore: 11 ft 4 in (3.45 m) (45cal) Total: 11 ft 9 in (3.58 m)[3] |
| Crew | 11[4] |
| Shell | Fixed QFHE 76.2 × 420 mm R[5][6] |
| Shell weight | 12.5 lb (5.7 kg), 1914; 16 lb (7.3 kg), 1916 |
| Calibre | 3 inches (76 mm) |
| Breech | Semi-automaticsliding-block[7] |
| Recoil | 11 inches (280 mm).hydro spring, constant[3] |
| Carriage | High-angle wheeled, static, or lorry mounting |
| Elevation | −10 – 90°[3] |
| Traverse | 360° |
| Rate of fire | 16–18 rpm[8] |
| 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[9] |
| Effective firing range | 16,000 ft (4,900 m)[8] |
| Maximum firing range | 23,500 ft (7,200 m), 12.5 lb shell[3] 22,000 ft (6,700 m), 16 lb shell[8] |
TheQF 3-inch 20 cwt anti-aircraft gun became the standard anti-aircraft gun used in the home defence of the United Kingdom against GermanZeppelins airships and bombers and on the Western Front inWorld War I. It was also common on British warships in World War I and submarines inWorld War II. 20cwt referred to the weight of the barrel and breech, to differentiate it from other 3-inch guns (1 cwt = 1hundredweight = 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 term3-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 by3-inch AA gun.
The gun was based on a prewarVickers naval 3-inch (76 mm) QF gun with modifications specified by theWar Office in 1914. These (Mk I) included the introduction of a verticalsliding 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] aWelin 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.
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 theQF 3.7-inch (94 mm) AA gun from 1938 onwards, but numbers of various Marks remained in service throughoutWorld War II. In Naval use it was being replaced in the 1920s by theQF 4-inch (100 mm) Mk V on HA (high-angle) mounting.
Britain enteredWorld 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 byRNVR crews, until the new specialised anti-aircraft version began production and entered service.[16] It was from then onwards operated byRoyal Garrison Artillery crews, with drivers and crew for motor lorries provided by theArmy Service Corps. However, the Mobile Anti-Aircraft Brigade based atKenwood 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 existing13-pounder and18-pounder guns proved inadequate, apart from theQF 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-altitudeZeppelins 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 correctfuze 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 againe 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 theQF 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 theWestern 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.
| 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[24] |
|---|---|---|---|---|---|---|
| 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)[25] |
| 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) |
At the beginning ofWorld 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 FactoryMaribyrnong, Australia.[26] Initially most were in the heavy anti-aircraft (HAA) role until replaced by the newQF 3.7-inch (94 mm) gun. Some were deployed as light anti-aircraft guns (LAA) for airfield defence, being transferred to theRAF Regiment when this was formed in 1942, until more40 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 theBritish Expeditionary Force in November 1939, compared with 48 of the modernQF 3.7-inch AA gun.[29]
In 1941, 100 of the obsolete guns were converted to become the3-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 toChurchill tanks to become the "Gun Carrier, 3-inch, Mk I, Churchill (A22D)".
InWorld War II the gun was carried byS-class,U-class andV-class submarines.
It was also fitted to older destroyers,A-class toI 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]
Britain supplied 24 Mk 3 guns and 7 M/34 mechanical fire control computers to Finland during theWinter War of 30 November 1939 to March 1940 but they arrived too late to be used. They were used during theContinuation War of 1941–1944.[31]
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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 |
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