The QF 4.5 inch gun has been the standard medium-calibre naval gun used by the Royal Navy as a medium-range weapon capable of use against surface, aircraft and shore bombardment targets since 1938. This article covers the early 45-calibre family of guns up to the 1970s. For the later unrelated 55-calibre Royal Navy gun, see 4.5 inch Mark 8 naval gun. Like all British nominally 4.5 inch naval guns, the QF Mk I has an actual calibre of 4.45 inches (113 mm). Background From the BL Mark I gun of 1916 the 4.7-inch (120 mm) calibre was the mid-calibre weapon of the Royal Navy, used particularly on destroyers. Apart from some ships armed with QF 4-inch Mk V guns due to shortages, it remained the standard weapon for destroyers up to the W-class destroyers of 1943. Its usefulness as an anti-aircraft weapon had been limited by the failure to develop a mounting with elevation over 55°, the lack of a predictive fire control system in destroyer classes built prior to the introduction of the 4.7 inch twin mount (see HACS), and the setting of fuzes by hand on early, pre-war, mountings. Later 4.7 inch mountings used mechanical fuze setters that were identical to those used on the 4.5 inch mountings. History The QF 4.5 inch L/45 was developed originally as a dual-purpose weapon with which to arm aircraft carriers and reconstructed battleships and battlecruisers. It was later developed as a new dual-purpose weapon with which to arm destroyers, supplanting the ubiquitous 4.7 inch gun. Despite the lower calibre, it had a heavier shell, resulting in a more powerful weapon. Variants The nomenclature system for guns used by the Royal Navy can be somewhat confusing. The gun and mounting each have their own Mark number and a letter(s) giving additional information. QF stands for quick firing, UD for upper deck, BD for between decks and CP for central pivot. QF Mark I: adopted after failure of a 5-inch gun project and used a fixed round, which proved to be somewhat heavy for the loaders to keep up the intended firing rate. Was fitted in twin mountings UD Mark III. QF Mark II: Land service used by the British Army. QF Mark III: same as Mark I, except for firing mechanism. Was fitted in twin mountings BD Mark II, BD Mark II** and BD Mark IV. HMS Illustrious fired about 3,000 rounds