The 4.5 inch (114 mm) gun was fitted to most Royal Navy ships as a medium range weapon capable of Surface, Anti-aircraft and Bombardment of Shore targets. Previously the main Destroyer gun had been the 4.7 inch (119 mm), but this had proven to be difficult to get serious elevations available for Anti-Aircraft use.

Introduced in 1943 on a trial basis, it was fitted either singly as a open mounting or as an enclosed twin turret. It was aimed by hand in the early days but later as Gunnery Radar and weapon control systems became more proficient it was aimed by the use of Radar with a Fire Control system below which consisted of either an analogue computer-for Surface fire-or a Predictor for Anti-aircraft fire. Later on in the 70's it was controlled by a digital computer, as it is to this day.

The early 4.5 inch (114 mm) gun required up to 30 people to operate a twin turret. This included the gunhouse, gunbay and magazine crew. In addition, the control team consisted of 14 to 20 people. The single 4.5 inch (114 mm) mounting deployed today is semi automatic and therefore needs fewer people to operate. There is no one in the gunhouse only a few in the gunbay loading the feed ring and the magazine crew. There are only about 3 people in the control team.

Ammunition was separate in the early mountings and turrets- that is the shell and propellant cartridge were handled separately. Nowadays the ammunition is fixed. The shell and cartridge are joined together. Ammunition is brought aboard via Ammunition Lighters in harbour and via Jackstay Transfer or Helo Transfer at sea. It is then manhandled to the hoists on the upperdeck and fed straight to the magazine or gunbay.

The weapon has undergone significant development over the years, with different marks being fitted to ships as follows

- initial phase the late war build types (but not Weapon class) and the Battle Class, single gun mounts except for the Battles.

- Second phase, Daring and County Class destroyers, Type 12, Type 12M, Type 41 and Type 61 frigates, always twin mounts.

- Third Phase Type 42 destroyers and Type 21, Type 22 and Type 23 frigates, always single mounts.

The third phase gun is a very different weapon and is derived from the Italian Otomat design, having only the bore in common with the previous phases.

The Type 81 or Tribal class frigates were an exception - they used mounts and guns from scrapped C class destroyers.