# 5-inch/54-caliber Mark 45 gun

The 5-inch (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.

## History

Development started in the 1960s as a replacement for the 5-inch (127 mm)/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 uses the Mark 45 with either the Mk 86 Gun Fire Control System or the Mk 34 Gun Weapon System.
Since before World War II, 5 inches (127 mm) has been the standard gun caliber for U.S. Naval ships. Its rate of fire is lower than the British 4.5 in (114 mm) gun, but it fires a heavier 5-inch (127 mm) 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

Mod 2 gun with a new control system. Never put into production.

### Mod 4

Receives a 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 captain, a panel operator, and four ammunition loaders, all located below decks. In fully automatic non-sustained firing operations (Mode IV), 20 rounds can be fired without any personnel inside the mount, using an autoloader.
In 2024-2025, U.S. Navy action in the Red Sea defending merchant ships and self-defense of U.S. Navy Destroyers saw increasing use of the Mk 45 gun against one-way attack drones, launched by the Houthi rebels from Yemen.

## Ammunition

Mark 68 HE-CVT
* Weight – 68.5 lb (31.1 kg)
* Projectile Length – 26.1 in (66.3 cm)
* Used only with Mods 0–2
Mark 80 HE-PD
* Weight – 67.6 lb (30.7 kg)
* Projectile Length – 26 in (66 cm)
* Explosive filler: 8.2 lb (3.7 kg) Composition B
Mark 91 Illum-MT
* Weight – 63.9 lb (29.0 kg)
* Projectile Length – 26.1 in (66.3 cm)
Mark 116 HE-VT
* Weight – 69.7 lb (31.6 kg)
* Projectile Length – 26 in (66 cm)
Mark 127 HE-CVT
* Weight – 68.6 lb (31.1 kg)
* Projectile Length – 26 in (66 cm)
Mark 156 HE-IR
* Weight – 69.0 lb (31.3 kg)
* Projectile Length – 26 in (66 cm)
Mark 172 HE-ICM (Cargo Round)
* Projectile Length – 26 in (66 cm)
* Used only with Mod 4

### Guided shell

In May 2014, the U.S. Navy released a request for information (RFI) for a guided 5-inch (127 mm) round that could be fired from Mark 45 guns on Navy destroyers and cruisers. This RFI came six years after the cancelation of the Raytheon Extended Range Guided Munition. The shell must have at least double the range of unguided shells for missions including Naval Surface Fire Support (NSFS)/Land Attack, and increasing anti-surface warfare (ASuW) capabilities against fast attack craft (FAC) and fast inshore attack craft (FIAC). The main purpose is to destroy incoming small boats at a greater range with a proximity fuse airburst blast fragmentation warhead to spray shrapnel over swarms.
Expected submissions include the BAE Systems Multi Service–Standard Guided Projectile (MS-SGP), Raytheon Excalibur N5, and OTO Melara Vulcano guided long-range projectile.
Naval Sea Systems Command is also looking to fire a version of the hyper-velocity projectile (HVP) developed for Navy electromagnetic railguns from conventional 5-inch deck guns. Using the HVP could give existing destroyers and cruisers better ability to engage land, air, and missile threats and allow more time to refine the railgun. The HVP would be a cheaper solution to intercepting incoming missiles than a missile interceptor costing hundreds of thousands of dollars. Converting the HVP to fire from conventional guns was not a program of record as of 2015.
HVP shells fired from 5-inch deck guns would travel at Mach 3, half the speed of a railgun but twice the speed of conventional rounds. The rounds would be more expensive than unguided shells but cheaper than missile interceptors, and engage air and missile targets out to 10–30 nautical miles (12–35 mi; 19–56 km).
During 2018 RIMPAC exercises, the USS Dewey (DDG-105) fired 20 HVPs from a standard Mk 45 deck gun; an HVP shell could cost US$75,000-$100,000, compared to $1-$2 million for missiles. The cost quoted here is an often-cited reference that continues to grow more stale as annual inflation calculations for Department of Defense are updated during the annual Pentagon Planning-Programming-Budgeting-Execution (PPBE) cycle that generates the annual defense budget request. The annual direction for inflation calculations is provided to the Department of Defense by the White House Office of Management and Budget (OMB) in circular A-11. A review of each fiscal year's inflation adjustment would be required to be applied to the circa 2019 'could cost' estimate noted above to approximate a current cost for procurement.
The HVP was renamed to the Gun-Launched Guided Projectile (GLGP) for the Electromagnetic Railgun project. In 2022, the United States Department of the Navy terminated the GLGP Research and Development effort.
In 2024, the U.S. Navy resumed developmental testing of the Hypervelocity Projectile (HVP) manufactured by BAE Systems USA. The HVP is a saboted round, and is also under development in a U.S. Army contract--the sabot size is larger for the Army version (155mm) but the projectile is common to the HVP developed for the U.S. Navy Mk45 guns, only in a smaller sabot (5"=127mm) for the Navy.

## Operators
### Current operators
#### Australia

* Royal Australian Navy
** Anzac-class frigate: Mod 2, upgraded Mod 2 versions fitted with a Common Control System.
** Hobart-class destroyer: Mod 4

#### Denmark

* Royal Danish Navy
** Absalon-class frigate: Mod 2

#### Greece

* Hellenic Navy
** Hydra-class frigate (MEKO 200 HN)

#### Japan

* Japan Maritime Self-Defense Force
** Mogami-class frigate: Mod 4
** Atago-class destroyer: Mod 4
** Maya-class destroyer: Mod 4
** Akizuki-class destroyer: Mod 4
** Asahi-class destroyer: Mod 4

#### South Korea

* Republic of Korea Navy
** Sejong the Great-class destroyer: Mod 4
** Chungmugong Yi Sun-sin-class destroyer: Mod 4
** Incheon-class frigate: Mod 4

#### New Zealand

* Royal New Zealand Navy
** Anzac-class frigate: Mod 2

#### Spain

* Spanish Navy
** Álvaro de Bazán-class frigate: Mod 2

#### Taiwan

* Republic of China Navy
** Kee Lung-class destroyer

#### Thailand

* Royal Thai Navy
** Naresuan-class frigate: Mod 2, upgraded to Mod 4

#### Turkey

* Turkish Navy
** Barbaros-class frigate (MEKO 200 TN II)
** Yavuz-class frigate (MEKO 200 TN I)

#### United States

* United States Navy
** Active service ships:
*** Ticonderoga-class cruiser: Mod 2
**** CG-52-73: Mod 4 after receiving the cruiser modernization
*** Arleigh Burke-class destroyer:
**** DDG 51–80: Mod 2
**** DDG 81–112: Mod 4
** Decommissioned:
*** California-class cruiser
*** Kidd-class cruiser
*** Spruance-class destroyer
*** Tarawa-class amphibious assault ship (later removed)
*** Virginia-class cruiser

### Future operators
#### Australia

* Royal Australian Navy
** Hunter-class frigate: 6 ships ordered

#### United Kingdom

* Royal Navy
** Type 26 frigate: 8 ships ordered, 3 in build (Mod 4)

#### Turkey

* Turkish Navy
** TF2000-class frigate: 8 in planned

### Cancelled deals

India
* Indian Navy: The mod 4 variant was planned to be deployed on 7 Nilgiri-class frigate and 4 Visakhapatnam-class destroyer. The procurement of 13 guns was cleared by the Indian Ministry of Defence in April 2018. Of the guns, 11 would be deployed on the above-mentioned ships while 2 would be in the INS Dronacharya missile and gunnery school, and the INS Valsura electrical and weapons engineering school. The US Defense Security Cooperation Agency cleared under the Foreign Military Sales for the same deal in 2019. The deal would be worth over $1 billion. In 2021, India moved on to the development of an indigenous gun of similar calibre due to cost and logistics factor. Till the completion of the development, the 11 destroyers and frigates would be fitted with already operational OTO Melara 76 mm naval gun. The 13 gun deal for the Indian Navy did not go through.