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° from horizontal, 13.7 seconds to reach 10,000 ft (3,000 m) at 40°, 18.8 seconds to reach 15,000 at 55°. This means that the gun team had to calculate where the target would be 9 – 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 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 mounted on heavy lorries, typically the Peerless 4 Ton. In addition, many were mounted on Royal Navy ships.
World War II
At the beginning of World War II in 1939, Britain possessed approximately 500 of these guns. Initially most were in the heavy anti-aircraft (HAA) role until replaced by the new 3.7 inch 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 40mm Bofors guns arrived However, it was discovered at mobilization that the 233 guns in HAA reserve were missing various parts and predicted fire instruments. 120 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 lb (5.7 kg) armour-piercing shell. They appear to have been mainly deployed in home defence.