QF 5.25 Inch Gun - Design

Design

Unlike the equivalent French design of 138 mm guns used on the Mogador class destroyers, the QF (quick-firing) 5.25 inch was designed to be a dual-purpose naval gun, for use against both ships and aircraft. Combining the secondaries and heavy anti-aircraft armament would allow a significant savings of weight for the King George V class of battleships, which were originally intended to meet the Washington Naval Treaty limit of 35,000 tons. The gun fired an 80 lb (36 kg) shell, considered the largest that a gun crew could handle easily enough to give the rate of fire needed for anti-aircraft use. The ballistic performances were very good, as the maximum range was in excess of 21,000 metres and the AP projectile could penetrate 3 in (76 mm) at around 9,500 metres. This compared well with the closest Italian design, the 135/45 mm, that was inferior almost in anything as power (range 19,600 metres with a 32.7 kg shell, 6 rounds per minute), and most important, it was not originally meant as dual purpuse weapon.

A class of anti-aircraft cruisers, the Dido class, was also designed using the gun as the main armament. 267 guns were built, making it the most numerous and important gun in the RN's dual purpose gun inventory. Not enough were available when the first Didos were launched for the full complement of ten guns; priority was given to the battleships. The Bellona class cruisers, a modification of the design of the Dido class, used a highly modified RP10Mk2 mount with Remote Power Control and much improved training and elevating speeds. The number of turrets was reduced from five to four, and the light AA numbers were increased.