QF 5.25 inch Mark I naval gun

QF 5.25 inch Mark I
5.25 in dual turret on HMS Sirius. A large number of empty cartridge cases stand on the deck in foreground, fired in support of the Allied invasion of Normandy, June 1944
Type	Dual-purpose gun
Place of origin	United Kingdom
Service history
In service	1940-1966 (naval) 1942-1960s (land)
Used by	Royal Navy Royal Artillery Royal Australian Artillery
Wars	World War II Korean War
Production history
Designed	1935
Variants	Mk I, Mk II
Specifications (bore length)
Weight	Barrel & breech: 9,616 lb (4,362 kg)
Length	Total: 22 ft 11.5 in (7 m)
Barrel length	Bore: 21 ft 10.5 in (6.67 m) L/50 (cal)
Shell	Separate QF, 80 pounds (36.29 kg) SAP or HE
Calibre	5.25-inch (133 mm)
Elevation	-5 to +70 degrees
Rate of fire	7-8 rpm sustained fire
Muzzle velocity	Naval: 2,672 ft/s (814 m/s) Army AA: 2,800 ft/s (850 m/s)
Effective firing range	Naval: 23,400 yd (21,400 m) at 45 degrees with HE shell at 2,600 ft/s (790 m/s) AA: 36,000 ft (11,000 m)
Maximum firing range	Naval: 24,070 yd (22,010 m) at 45 degrees with HE shell at 2,672 ft/s (814 m/s) AA: 46,500 ft (14,200 m)

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.

Unlike its French and Italian contemporaries of similar size the QF 5.25 was designed as dual-purpose gun capable of engaging both aircraft and surface targets. Combining the secondary and heavy anti-aircraft armament allowed a significant weight savings for the King George V class battleships, which were designed to meet the Washington Naval Treaty limit of 35,000 tons. The gun fired an 80 lb (36 kg) 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 was carried in Mk I twin mountings by the King George V class and in Mk II twin mountings on 9 of the first 11 Dido class anti-aircraft cruisers, the exceptions being HMS Scylla and HMS Charybdis which mounted QF 4.5 in Mk.III guns due to shortages of the 5.25 inch gun. The last 5 of the Dido class also known as the Bellona-class anti-aircraft cruisers mounted the 5.25 inch 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 HMS Anson. The last of the RN's battleships which was commisioned after the war, HMS Vanguard, featured Mk 1* 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.