The Mark 45 gun 5″ (127 mm)/54 caliber (Mk 45) lightweight gun is a U.S. naval artillery gun mount consisting of a 5 in (127 mm) 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.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 US Navy (DDG 51-80 / CG’s)Mod 3: same gun with a new control system; never put into production.Mod 4: longer 62 caliber barrel for more complete propellant combustion, higher velocity, greater range at +20 nm (36 km) and thus more utility for land attackFully-automatic naval gun mount employed against surface (Anti-Surface Warfare – ASuW), air (Anti-Air Warfare – AAW), and land attack (Naval Surface Fire Support – NSFS) targets.The MK 45 gun was developed as a lighter weight, more easily maintained replacement for the MK 42 5”/54 caliber gun mount. It is designed to engage surface and air targets and to provide naval surface fire support for expeditionary operations. The MK 45 Mod 4 gun mount upgrade includes a longer barrel (62 caliber) that improves the gun’s effectiveness as a land attack weapon (naval surface fire support).The gun mount includes a 20 round automatic loader drum. The gun’s maximum firing rate is 16-20 rounds from the loader drum per minute. The rounds in the loader drum can be fired with one crewmember located at the EP-2 console below deck. Additional rounds can be accomplished by the full crew (Mount Captain, EP-2 Operator and four ammunition handlers), all of which are stationed below deck.