The BL 16 inch Mark I was a British naval gun introduced in the 1920s and used on the two Nelson-class battleships. The barrel was 45 calibres long ("/45" in shorthand) meaning 45 times the bore (16 in) - long. These wire-wound built-up guns had originally been planned for the cancelled G3-class battlecruiser design upon which the Nelson class drew. An improved weapon, the BL 16 inch Mark II was designed for the cancelled Lion-class battleships; only four guns and no turrets were produced.