BL 4-inch Mk IX naval gun

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

World War I


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 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". Guns were thereafter used in single-gun mountings, typically on smaller ships as the main armament.

World War II
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 (K181), 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.