BL 4 inch Mk IX naval gun
|Ordnance BL 4 inch gun Mk IX|
On Flower-class corvette HMCS Calgary in World War II
|In service||1916 - 1945|
|Used by||South African Navy|
|Wars||World War I
World War II
|Weight||2 tons barrel & breech|
|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|
|Rate of fire||10-12 rpm|
|Muzzle velocity||800 metres per second (2,600 ft/s)|
|Maximum firing range||12,660 metres (13,850 yd)|
The BL 4-inch gun Mk IX[note 1] was a British medium-velocity naval gun introduced in 1916 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.
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 primary armament.
World War II
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.
- 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
- 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.
- Campbell, Naval Weapons of WWII, p.42-43.
- Campbell, Naval Weapons of WWII, p.38.
- Jane's Fighting Ships 1919, page 62
- Tony DiGiulian, British 4"/45 (10.2 cm) BL Marks IX and X
- Campbell, John (1985). Naval Weapons of World War Two. Naval Institute Press. ISBN 0-87021-459-4.
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