BL 4-inch Mk I – VI naval gun

The BL 4-inch gun Mk I – Mk VI were a family of early British breech-loading 4-inch naval guns.

13 cwt Mk I, 15 calibres
This was the first 4-inch BL gun. With its short (60-inch total) barrel it had a range of only 5,500 yards.

22.5 cwt Mk I, 25 calibres
With its longer barrel (100 inch bore) Mk I had a range of 7,200 yards. Both early Mk I types were quickly withdrawn from service following the explosion of a similar BL 6 inch Mk II gun on board HMS Cordelia in June 1891. Mk I 22.5 cwt continued to be used for training.

26 cwt Mks II - VI, 27 calibres
The improved 27-calibre Mk II gun and subsequent Marks, often referred to as 4 inch 26 cwt, replaced the early Mk I versions in service. The longer barrel (108 inch bore : 27 calibres) gave it a range of 7,700 yards.

Mk II guns and later Marks armed the following warships :
 * HMS Inflexible (1876) as re-gunned in 1885
 * HMS Bellerophon (1865) as re-gunned in 1885
 * Pygmy-class composite screw gunboats of 1888
 * Redbreast-class gunboats launched in 1889
 * HMS Alexandra (1875) as re-gunned in 1891
 * Alert-class sloops of 1894

The gun was succeeded in its class from 1895 by the QF 4-inch gun Mk I.

QFC 4-inch gun
A small number of these guns were converted to QF to use the same cartridges as the QF 4-inch gun. They were designated Mk I/IV, I/VI etc. depending on which Mark of BL 4-inch had been converted. All had a bore of 27.85 calibres after conversion, with a muzzle velocity of 2,177 ft/second.

Surviving examples

 * A gun from 1888 at Explosion! The Museum of Naval Firepower, Gosport from Victorian Forts and Artillery website
 * A gun from HMS Gannet, mounted on top of Calshot Castle at the entrance to Southampton Water from Victorian Forts and Artillery website