Ordnance, QF 4.5in Gun Mark II (Anti-Aircraft)

The Ordnance, 4.5in Gun, was a heavy anti-aircraft gun produced for use on static sites before the Second World War, and which remained in service throughout the war, although many were converted into the higher altitude 3.7in Gun Mk VI later in the war.

During the 1920s and early 1930s the Army had wanted a 4.7in (120mm) anti-aircraft gun to be used to defence important positions, but the money wasn’t available to develop this alongside the 3.7in anti-aircraft gun.

In the 1930s the need for a heavy AA gun was becoming more urgent, and it was decided to use an existing Navy 4.5in gun. This existing gun had similar ballistics to those desired in the 4.7in gun, already existed, ammo was available, and as it was largely intended for use near naval bases and docks around the vulnerable coastline the Army would be able to use the Admiralty supply chain.

The Army version became the 4.5in Gun Mk 2. It was carried on a pedestal that rotated on its base. In use the base would be bolted onto a concrete emplacement. To move the gun wheels could be attached to the pedestal base to create a travelling platform, but it wasn’t meant to be fired from this.

The 4.5in gun underwent trials in 1936, and was approved for issue in September 1937. Production was underway by October 1937. The first 4.5in gun was officially emplaced in February 1939 at one of the first new permanent heavy anti-aircraft sites that were under construction.

The gun was initially manually operated. In 1940 an electric rammer was added to help with the 54lb shells, and this increased the rate of fire to 8 rounds/ minute. It had a maximum ceiling of 44,000ft and an effective ceiling of 34,500ft (the height at which it could fire at a target approaching at 400mph for 20 seconds before elevation reached 70 degrees).

Guns mounted close to the coast were often installed on Mounting Mark 1A, which allowed elevation to go down to -9.5 degrees to allow it to fire down onto nearby naval targets. It had a maximum horizontal range of 22,800 yards. Coastal defence guns were given armour piercing shells as well as the normal high explosive AA shells.

By June 1942 98 sites were equipped with the 4.5in gun, at London, Portsmouth, Southampton, Thames & Medway, Cardiff, Manchester, Sheffield, Leeds, Birmingham, Scapa, the Mersey, the Humber, the Forth, the Tyne, the Tees and the Clyde. Most sites contained four guns, with some of the London sites having eight.

During 1942 work began on converting the 4.5in guns into high velocity 3.7in guns, by lining the barrels down to the lower size but keeping the original 4.5in charge. Once this had been proven, a large number of existing 4.5in guns were converted to the new 3.7in Gun Mk VI in their gun emplacements. This started in August 1943 at AA Command’s user trials site at Bell Farm in the Thames and Medway gunnery area. Between October and November 1943 almost 100 4.5in guns in the inner zone were converted, and work them moved outwards. By the end of the war 343 of AA Command’s 4.5in guns had been converted, leaving only 63 as 4.5in guns.

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