Ordnance, QF 5.25in AA Gun

The Ordnance, QF, 5.25in AA Gun, saw a naval 5.25in gun intended for use on cruisers converted into a land based anti-aircraft gun, entering service from 1942 onwards with Anti-Aircraft Command.

In January 1941 the War Office issued a specification for a new anti-aircraft gun, after the performance of German aircraft began to outpace the existing 3.7in AA gun. The new gun should be able to fire a shell to 50,000ft in 30 seconds and fire three rounds and have the fourth in the chamber in 20 seconds.

Four alternatives were considered – one involving combining the existing 4.5in AA gun with a 3.7in barrel, the other three using the Naval 5.25in dual purpose gun. Of these one was to simply use the 5.25in gun, the others were to use the 5.25in chamber and charge with either a 4.5in or 3.7in barrel, so the extra charge from the larger 5.25in shell would propel the smaller projective at a much higher speed. Both alternatives were much heavier than the 3.7in Gun, so would be used from fixed positions.

It was decided to use the 5.25in gun as the long term solution and the 4.5in gun relined to 3.7in as a temporary solution. The Navy provided three twin mountings of their version of the gun in 1942. After successful trials it was decided to produce two single gun mountings for this gun.

These three twin mounts were installed in London by the spring of 1942 as part of the active air defences of the city, and remained the only 5.25in AA guns in use with AA Command for almost a year. Ironically these guns then found themselves largely without targets, as the Luftwaffe didn’t return to London in strength until the winter of 1944-45. However they were able to prove that the gun was a suitable and powerful AA gun.

The Mk 1A would be a dedicated anti-aircraft gun. It would he mounted in a circular concrete emplacement, with an underground room containing a diesel generator and hydraulic pump. The gun would be protected by an open backed mild steel shield.

The Mk 1B was for use as a dual purpose anti-aircraft and coast defence gun, and was an armoured turret. Once this was installed it was a low turret, fully enclosed on the sides and rear, but with a gap in the front to allow for the full range of elevation of the gun. There was as much below ground as above, with the gun supported on a roller race and requiring a significant amount of work to build the full emplacement. Each contained space for 324 shells and 288 charges in an ammunition gallery built around a circular walkway at the top of the emplacement.  

The first single barrel 5.25in guns reached AA Command in the spring of 1943, and work on their new gun sites in the summer. The first guns went to the inner gun zone around London, with the first sixteen going to Hackney Marshes, Bostall Heath, Dulwich and Wormwood Scrubs. By the end of July the first batch of 40 had arrived, with most posted to London and some to the eastern approaches. They were used to replace existing 3.7in or 4.5in guns on permanent AA sites. By mid August sixteen guns had been allocated to the Humber and eight to the Mersey. A large number went to the Tyne. During 1944 5.25in guns also went to sites at Portsmouth, Plymouth and the Isle of Wight. However by the time they were entering service, the threat from German bombers had largely gone, and only 112 5.25in guns were actually emplaced by VE Day, and of these only 48 were officially operational.

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