The 5" /54 caliber 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 Mark 42 5"/54 caliber gun (127mm) is a naval gun mount used by the United States Navy and other countries. It consisted of the Mark 18 gun and Mark 42 gun mount. United States naval gun terminology indicates the gun fires a projectile 5 inches (127.0 mm) in diameter, and the barrel is 54 calibers long In the 1950s a gun with more range and a faster rate of fire than the 5"/38 caliber gun used in World War II was needed, therefore, the gun was created concurrently with the 3"/70 Mark 26 gun for different usages. The 5"/54 Mk 42 is an automatic, dual-purpose gun mount. It is usually controlled remotely from the Mk 68 Gun Fire Control System, or locally from the mount at the One Man Control (OMC) station.