BL 16-inch Mk I naval gun
The BL 16-inch Mark I was a British naval gun introduced in the 1920s and used on the two Nelson-class battleships. A breech-loading gun, the barrel was 45 calibres long ("/45" in shorthand) meaning 45 times the 16-inch (406 mm) bore – 60 ft (18 m) long.
Source: Wikipedia — BL 16-inch Mk I naval gun (CC BY-SA 4.0)