4.5"-55 caliber Mark 8 gun
The (British) Royal Navy's standard medium naval gun, introduced in 1938, is the 4.5 inch (actually 4.45 in/113 mm); a 55 caliber Mark 6 was standard for many years. In 1966, the Mark 8 project was started, with the goal of achieving a higher rate of fire.
The initial Mod 0 redesign, derived from a land 105mm howitzer, first was deployed on an Iranian ship in 1971. During the Falklands War, the Mark 8 Mod 0 suffered more jamming problems than its predecessor, especially on the Type 21-class and Type 22-class frigates. 
The jamming problems were fixed in the Mark 1 by replacing fragile hydraulic actuators with electric motors. The electric motors also reduce weight and power.
Other improvements in the Mod 1 include a new housing with a reduced radar cross-section.