US Navy's destroyer USS Ralph Johnson conducts training in Philippine Sea.

According to information published by the US DoD on July 1, 2023, the Arleigh Burke-class destroyer USS Ralph Johnson (DDG 114) engaged in a live-fire training session within the Philippine Sea.
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5"/54 caliber Mark 45 lightweight gun of the destroyer USS Ralph Johnson. (Picture source: US DoD)

The 127mm (5")/54 caliber (Mk 45) lightweight gun is a naval artillery mount employed by the United States. Manufactured by United Defense, later acquired by BAE Systems Land & Armaments, it consists of a 127mm (5 in) L54 Mark 19 gun mounted on the Mark 45 platform. BAE Systems has continued the production of this powerful weapon system.

In the 1960s, a development project was initiated to replace the existing 127mm (5")/54 caliber Mark 42 gun system, which was first introduced in 1953. The goal was to create a lighter and more easily maintainable gun mount.

The United States Navy adopted the result of this project, known as the Mark 45, and integrated it with either the Mk 86 Gun Fire Control System or the Mk 34 Gun Weapon System.

The caliber of 127mm (5 inches) has been the standard for U.S. Naval ships since before World War II. While its rate of fire is slower compared to the British 4.5 in (114 mm) gun, the 127mm (5-inch) shell it fires carries a larger burst charge, increasing its effectiveness against aircraft.

Key specifications of the 127mm (5")/54 caliber (Mk 45) lightweight gun include a mass of 21,691 kg (47,820.5 lb), a length of 8.992 m (29 ft 6.0 in), and a barrel length of 6.858 m (22 ft 6.0 in).

The rifling spans 5.82 m (19 ft 1 in), or 7.874 m (25 ft 10.0 in) for the Mod 4 variant. The gun employs 127 x 835mm .R shells, with a conventional shell weighing 31.75 kg (70.0 lb).

The gun mount offers an elevation range of -15° to +65°, with a maximum elevation rate of 20°/s. Its traverse capability spans ±170° from the centerline, with a maximum traversing rate of 30°/s.

The 127mm (5")/54 caliber (Mk 45) lightweight gun boasts an automatic firing rate of 16-20 rounds per minute. The muzzle velocity reaches 2,500 ft/s (760 m/s) for standard rounds, while reduced charge rounds for defilade fire or illumination purposes achieve a muzzle velocity of 1,500 ft/s (460 m/s). The effective firing range is approximately 13 nautical miles (24.1 km) or 20 nautical miles (37.0 km) for the Mod 4 variant.