An introduction to the 3.7 inch Anti-Aircraft gun

Artillery on Parade at Fort Nelson takes place this coming weekend. The event will feature firings of artillery from the First and Second World Wars, plus a live-action re-enactment of an air attack and a flyover by a Messerschmitt BF-108. Here Phil Magrath, Curator of Artillery at Fort Nelson, explores one of the key exhibits over the weekend, the 3.7-inch Anti-Aircraft gun. 

Argumentative ballisticians could easily engage themselves in vigorous debate about the relative merits of Britain’s foremost counter to German aerial bombardment during the Second World War, the 3.7-inch Anti-Aircraft gun and the Third Reich’s equivalent, the 88mm Flak 18/36/37. Broadly speaking the differences were not great, both could boast excellent performance especially the latter when it was deployed with great success in an anti-tank role.

The former was introduced into British service in 1937 replacing the 3-inch gun of 20 hundredweight – a relic from the First World War. Designed and constructed by Vickers-Armstrong Ltd it was an extremely advanced and intricate weapon for its time despite the large and weighty carriage that exceeded one of the original design criteria of an all-up 8 tons weight. This relative lack of mobility together with its impressive ability to send a 28 lb High explosive shell to an effective ceiling of 32,000 feet  made its selection to become the main weapon in the country’s Second World War anti-aircraft defence an easy one. Improvements to the system were effected on a consistent basis, the principle one involving the adoption of the Molins Fuze Setter No.11. This device, invented by the Molins Machinery Company who before the war manufactured cigarette-making machinery, was a combined automatic  fuze-setter and loading mechanism which considerably lightened the work of the detachment, speeding-up the firing rate and quantifying a fixed ‘dead-time’ between fuze-setting and gun- firing which could be taken into account in the prediction process.

anti aircraft
3.7-inch Anti-Aircraft Gun, Mark II, dated 1943 AT Fort Nelson (XIX.840)

A further significant improvement came in 1943 with the introduction of the Mark 6. This gun achieved an effective ceiling of 50,000 feet, a time of flight to that altitude of 30 seconds and the ability to fire three rounds with a fourth loaded and ready in 20 seconds. Since such a high muzzle velocity was required to reach this altitude a new system of rifling was designed by Colonel G.O.C . Probert and became known as Research Department Rifling. This used a combination of variable groove depth, a smooth-bore section, an improved driving band and twin centring bands on the projectile shoulder. So successful was this gun that it remained in service long after the end of the Second World War and not declared obsolete until 1959.

Fort Nelson has a direct link with the 3.7 since a series of ten ammunition stores were constructed across the Parade in 1939 – two of which have survived – for the storage of its ammunition as well as for that of the 5.25-inch Anti-Aircraft gun.

Operating these guns and others in the defence of Portsmouth and its surrounding district was 57th Heavy Anti-Aircraft Regiment Royal Artillery and in 1941, for example, there were sixteen 3.7’s emplaced in either a static – that is fixed – or mobile batteries. Not far from Fort Nelson was just such a fixed battery of four 3.7’s at Monument Farm, Swivelton Lane in a position that commanded an excellent field of fire over Portsmouth and Gosport. Fortunately the Fort and its ammunition despite near misses was not hit during the major Luftwaffe raids on Portsmouth. On the 10th January 1941 around 300 aircraft dropped an estimated 25,000 incendiaries and hundreds of High Explosive bombs leaving vast swathes of the City destroyed and leaving one-hundred and seventy people killed and over four-hundred injured.

Over the weekend of July the 22nd and 23rd Royal Armouries at Fort Nelson holds its annual ‘Artillery on Parade’ event. This year the theme is predominantly Second World War and the key visiting exhibits will be two 3.7-inch guns and three working searchlights utilised in an air-raid re-enactment.

Tickets to the event are available   online at: