The QF 5.25-inch Mark I gun was the heaviest dual-purpose gun used by the Royal Navy during the Second World War. Although considered less than completely successful, it saw extensive service. 267 guns were built. Design Unlike its French and Italian contemporaries of similar size the QF 5.25-inch gun was designed as dual-purpose equipment capable of engaging both aircraft and surface targets. Combining the secondary and heavy anti-aircraft armament allowed a significant weight savings for the s, which were designed to meet the Washington Naval Treaty limit of 35,000 tons. The gun fired an shell, which was considered the largest that a gun crew could easily handle while still having the rate of fire needed for anti-aircraft use. In 1944, VT-fuzed shells became available, making the gun significantly more effective against aircraft. The 5.25-inch gun was carried in Mk I twin mountings by the King George V class and in Mk II twin mountings on nine of the first eleven anti-aircraft cruisers, the exceptions being and , which mounted QF 4.5-inch Mk III guns due to shortages of the 5.25-inch gun. The last five of the Dido class also known as the Bellona-class anti-aircraft cruisers mounted the 5.25-inch gun in the Remote Power Control RP10 Mk II mountings, which offered much-improved training and elevating speeds. The number of turrets on the Bellona class were reduced from five to four and the number of light AA guns were increased. The RP10 Mk II mountings were also later used to replace the Mk I mountings on . The last of the Royal Navy's battleships which was commissioned after the war, , had MK I turrets with enlarged gun houses compared to those fitted to the King George V and Dido-class ships, although the width between the guns remained the same. A slightly more powerful Mk II variant was also deployed by the British Army as an anti-aircraft and coast-defence gun. Naval service The RN Gunnery Pocket Book published in 1945 states: they fulfil the combined functions of H.A. Long Range Armament and Secondary Armament against surface craft. The 5.25 in. calibre with separate ammunition is used for dual High Angle and Low Angle Armament, since it gives the reasonable maximum weight of shell which can be loaded by the average gun's crew for sustained periods at all angles of elevation. The maximum rate of fire should be 10-12 rounds per minute.Sired, Enemy