BL 4-inch Mk IX naval gun

The BL 4-inch Mk IX naval gun^{[note 1]} was a British medium-velocity naval gun introduced in 1917 as secondary armament on the Renown-class battlecruisers and Glorious-class "large light cruisers", but which served most notably as the main armament on Flower-class corvettes throughout World War II.

Ordnance BL 4-inch Mk IX gun
On Flower-class corvette HMCS Calgary in World War II
Type	Naval gun
Service history
In service	1917–1945
Used by	Royal Navy Royal Canadian Navy Free French Navy Hellenic Navy Royal Indian Navy Netherlands Navy Royal New Zealand Navy Norwegian Navy South African Navy
Wars	World War I World War II
Production history
No. built	2,382[1]
Specifications
Mass	2 tons barrel & breech[2]
Barrel length	180 inches (4.572 m) bore (45 calibres)
Shell	31 pounds (14.1 kg)
Calibre	4 inches (101.6 mm)
Breech	Welin interrupted screw
Elevation	-10 degrees to +30 degrees[3]
Rate of fire	10-12 rpm[3]
Muzzle velocity	800 metres per second (2,600 ft/s)[2]
Maximum firing range	12,660 metres (13,850 yd)[2]

History

World War I

Original aft triple mounts on HMS Repulse c. 1916–1917

The gun was based on the barrel of the QF 4-inch Mk V and the breech mechanism of the BL 4-inch Mk VIII^[4] and was first introduced in World War I on capital ships as secondary armament in triple-gun mountings, intended to provide rapid concentrated fire. This turned out to be unworkable in practice. Jane's Fighting Ships of 1919 commented, "4-inch triples are clumsy and not liked. They are not mounted in one sleeve; have separate breech mechanism, a gun crew of 23 to each triple".^[5] Guns were thereafter used in single-gun mountings, typically on smaller ships as the main armament.

World War II

Coast defence gun and crew at Fort Crosby near Liverpool, UK, August 1940

Cleaning the breech on transport St Essylt, Suez 1942

In World War II, the gun was employed on many small warships such as Flower-class corvettes and minesweepers, primarily for action against surfaced submarines.

This was the last BL 4 inch gun in British service: all subsequent guns have used charges in metal cartridges "QF". It was succeeded on new small warships built in World War II by the QF 4-inch Mk XIX gun which fired a slightly heavier shell at much lower velocity and had a high-angle mounting which added anti-aircraft capability.

Surviving examples

• On board HMCS Sackville, the last surviving Flower-class corvette, at Halifax, Nova Scotia, Canada
• A gun at the entrance to the marina in Hull, UK
• A gun at Port Isaac, Cornwall, UK
• Leith Harbour, South Georgia.

See also

• List of naval guns

Notes

1. Mk IX = Mark 9. Britain used Roman numerals to denote Marks (models) of ordnance until after World War II. Mark IX indicates this was the ninth model of BL 4-inch gun.