BL 12 inch naval gun Mk I – VII

The BL 12 inch naval gun Mk I was Britain's first attempt to match the large breech-loading naval guns being installed in rival European navies, particularly France. Mks I - VII all had a barrel of approximately 303 inches in length (25.25 calibres) and similar performance.

Naval service
Mk II was installed on battleships Colossus from 1882, Collingwood and the coastal service ironclad Conqueror. Mk II guns failed in service and were quickly replaced by Mks III. IV and V, with many changes and improvements. The later marks were also mounted on Hero, sistership of Conqueror, and on Edinburgh, sistership of Colossus.

Coast defence gun
Mks I, VI and VII were coast defence versions with trunnions for mounting on recoil slides. They were installed in forts in England at Spitbank Fort, No Man's Land Fort and Horse Sand Fort from 1884 onward and were in active service during World War I.