The BL 6-inch Mark XII naval gun[8] was a British 45 calibre naval gun which was mounted as primary armament on light cruisers and secondary armament on dreadnought battleships commissioned in the period 1914–1926, and remained in service on many warships until the end of World War II.

This was a high-velocity naval gun consisting of inner "A" tube, "A" tube, wound with successive layers of steel wire, with a jacket over the wire.

It was superseded as secondary armament on new battleships in the 1920s by the 50-calibre 6-inch Mk XXII gun, and as main armament on new light cruisers in the 1930s by the 50-calibre 6-inch Mk XXIII gun.

[10] This gun generated a higher pressure in the chamber on firing compared to preceding 6-inch guns such as Mk VII and Mk XI.

This necessitated use of special shells capable of withstanding a pressure of 20 tons per square inch on firing, which had "Q" suffixed to the name.