Ordnance, QF, 3.7in Gun Mark 6

The Ordnance, QF 3.7in Gun Mark 6 fired a 3.7in shell with a 4.5in charge, to produce a gun with a higher effective ceiling than the standard 3.7in anti-aircraft gun.

By 1941 the increased performance of German aircraft was starting to outclass the 3.7in gun. In January 1941 the War Office requested a new gun that could fire a shell to 50,000ft in 30 seconds, fire three rounds and have the fourth in the chamber in 20 seconds. Four solutions were considered, three involving modifications to a 5.25in naval gun and the fourth reducing the bore of the existing 4.5in anti-aircraft gun to 3.7in but still using the 4.5in charge. Once of the 5.25in solutions was picked as the long term answer, with the modified 4.5in gun as an interim solution.

An experimental gun was built as the 3.7in Gun Mark 5. This involved lining the barrel of a 4.5in gun down to 3.7in, and using it to fire 3.7in shells with a 4.5in cartridge. This gun was tested at Shoeburyness in the early autumn of 1942. The greatly increased charge did indeed improve the ceiling of the weapon, but it also caused so much wear that the barrels would have had to be replaced after almost every raid.

The solution was developed by Colonel Probert of the Armaments Research Department. He developed a new barrel with rifling grooves that got increasingly shallow along the barrel, disappearing three calibres from the muzzle. This was combined with a special shell that had a ‘centring band’ on the shoulder of the shell as well as the normal ‘driving band’ near the base of the shell. These soft metal raised bands would both engage with the rifling, and would be squeezed down as they passed along the barrel, reducing wear on the barrel lining itself. This was tested and found to be a success. Muzzle velocity was raised from 2,600ft sec on the original 3.7in gun to 3,470ft/ sec

As a result the new design entered production as the Ordnance, QF, 3.7in Gun Mk 6. It was much heavier than the normal 3.7in gun, so was only installed on fixed mounts. However it was also more effective, capable of firing its 28lb shells to a maximum ceiling of 59,300ft and an effective ceiling of 45,000ft, making it by far the best anti-aircraft gun of similar calibres (such as the German 88mm guns). Indeed it had a better effective ceiling that the 5.25in gun.

It was also possible to convert guns that were already in static positions. By the end of November almost 100 of the 4.5in guns in the London defences had been converted from 4.5in to 3.7in Mk 6 standard, and work them moved onto other gunnery sites. By the end of the war 343 of AA Command’s 4.5in guns had been converted, leaving only 63 in their original form.

Although the Mk 6 was meant to be a temporary design, it ended up remaining in service well into the post-war period. It became the basis of ‘Longhand’, a belt fed 3.7in anti-aircraft gun that was developed in the 1950s, with a rate of fire of 80 rounds/ second! The standard Mk 6 remained in service until 1959, when it was finally replaced by anti-aircraft missiles.

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