The 127 mm (5")/54 caliber (Mk 45) lightweight gun is a U.S. naval artillery gun mount consisting of a L54 Mark 19 gun on the Mark 45 mount. It was designed and built by United Defense, a company later acquired by BAE Systems Land & Armaments, which continued manufacture. The latest 62-calibre-long version consists of a longer-barrel L62 Mark 36 gun fitted on the same Mark 45 mount. The gun is designed for use against surface warships, anti-aircraft and shore bombardment to support amphibious operations. The gun mount features an automatic loader with a capacity of 20 rounds. These can be fired under full automatic control, taking a little over a minute to exhaust those rounds at maximum fire rate. For sustained use, the gun mount would be occupied by a six-person crew (gun captain, panel operator, and four ammunition loaders) below deck to keep the gun continuously supplied with ammunition. History Development started in the 1960s as a replacement for the 127 mm (5")/54 caliber Mark 42 gun system that had debuted in 1953 with a new, lighter, and easier-to-maintain gun mounting. The United States Navy use the Mark 45 with either the Mk 86 Gun Fire Control System or the Mk 34 Gun Weapon System. Since before World War II, 127 mm (5 inches) has been the standard gun caliber for U.S. Naval ships. Its rate of fire is lower than the British gun, but it fires a heavier 127 mm (5-inch) shell which carries a larger burst charge that increases its effectiveness against aircraft. Variants Mod 0: used mechanical fuze setter. Two-piece rifled construction, with replaceable liner Mod 1: electronic fuze setter replaces the mechanical one. Made with a unitary construction barrel, which has a life span approximately twice that of the Mark 42 gun. Mod 2: export version of Mod 1, but now used in the U.S. Navy Mod 3: same gun with a new control system; never put into production Mod 4: longer 62-caliber barrel (versus Mod 1 and 2's 54 caliber) for more complete propellant combustion and higher velocity and thus more utility for land attack. Was designed to use the Mark 171 Extended Range Guided Munition (ERGM), which was canceled. The Mk 45 mod 4 uses a modified flat-panel gun turret, designed to reduce its radar signature. In sustained firing operations (Mode III), the gun is operated by a six-person crew: a gun