Mk 38 Mod 4 Gun Weapon System

History

The Mk 38 Mod 4 Gun Weapon System is the latest U.S. Navy development of the Mk 38 shipboard gun family. It replaces the earlier 25 mm armament with a 30 mm Mk 44 Bushmaster II chain gun and combines the weapon with improved sensors, fire control and integration with the Aegis Combat System.

The Mod 4 was developed to improve defense against unmanned aerial systems, fast and maneuverable unmanned surface vessels, and other close-range surface threats. It can also carry an optional 12.7 mm M2HB heavy machine gun mounted coaxially with the main gun, giving the operator an additional weapon for engaging selected targets.

The system represents a substantial redesign compared with earlier Mk 38 versions. According to NAVSEA, Naval Surface Warfare Center Dahlgren Division served as technical design agent for the Mod 4 and led its integration with Aegis.

Development and testing

The Mk 38 family originated in the late 1970s, when the U.S. Navy developed a replacement for the older 20 mm Mk 16. Early Mk 38 systems used the 25 mm M242 Bushmaster chain gun. Later Mod 2 and Mod 3 versions introduced remote control, stabilized mounts, electro-optical and infrared sensors, laser ranging, improved ammunition handling and increasingly capable surveillance systems.

The Mk 38 Mod 4 moved to the 30 mm Northrop Grumman Mk 44 Bushmaster II and incorporated the Mk 48 Mod 2 electro-optical sight system and Mk 134 Mod 0 operator console. The 30 mm gun is capable of using air-burst ammunition. The Mod 4 configuration is based on the MSI Defence Systems Seahawk DS30M A2 mounting, with adaptations for U.S. Navy requirements including the coaxial .50-caliber weapon option and changes to the power system architecture.

In July 2022, a complete Mk 38 Mod 4 system was tested at the Potomac River Test Range operated by Naval Surface Warfare Center Dahlgren Division. During the event, the system identified, tracked and engaged fixed and moving surface and aerial targets with live ammunition. This test demonstrated the complete detection, fire-control and engagement chain. MSI Defence Systems subsequently supplied eight prototype systems for test and qualification activities.

Low-rate initial production contracts were awarded in September 2023. One contract funded 15 Mk 88 Mod 4 gun mountings together with associated hardware and spares, while a separate contract covered Mk 48 Mod 2 electro-optical sight systems, spares, repairs and transportation cases.

First ship installation

USS Mustin (DDG 89), an Arleigh Burke-class Flight IIA guided-missile destroyer, became the first U.S. Navy warship fitted with the Mk 38 Mod 4. The installation was completed during a maintenance and modernization period in San Diego and was publicly reported in April 2024. The ship received a twin-mount installation consisting of Mk 88 Mod 4 gun mounts, the separately mounted Mk 48 Mod 2 electro-optical sight system and the Mk 134 Mod 0 operator console.

In February 2026, NAVSEA confirmed that USS Mustin was the first Navy warship to receive the upgrade and stated that Dahlgren Division was finalizing the configuration for introduction to additional platforms. Photographs released in August and September 2026 showed Mustin operating and firing the system during training in the Indian Ocean.

Design

The Mk 38 Mod 4 is a remotely operated naval gun system intended for close-range ship defense. The system can be controlled through a combat management system or through an electro-optical fire-control system using a remote independent sight. MSI Defence Systems US describes the design as a fully integrated naval gun with modular open architecture.

Gun mount and armament

The principal weapon is the Mk 44 Bushmaster II 30 mm chain gun. It supports single-shot and burst fire as well as low and high automatic rates of approximately 100 and 200 rounds per minute. The gun can employ air-burst ammunition, expanding its usefulness against aerial targets.

An optional 12.7 mm M2HB heavy machine gun can be mounted coaxially above the 30 mm weapon. In this configuration the two weapons form the distinctive piggy-backed arrangement visible on USS Mustin. The M2 has a rate of fire of approximately 500 rounds per minute. The secondary weapon gives operators another engagement option without requiring a separate remotely controlled mount.

The Mk 88 Mod 4 gun mounting is derived from the Seahawk DS30M A2. Its structure and servo systems are optimized for accuracy at extended engagement distances. The U.S. Navy configuration also incorporates a revised power-system architecture.

Sensors and fire control

The Mk 48 Mod 2 electro-optical sight system is mounted separately from the gun. It is fully stabilized on two axes and contains television and thermal imaging cameras together with a high-refresh-rate laser rangefinder. Separating the sight from the weapon allows the sensor and gun installation to be arranged independently on the ship.

The Mk 134 Mod 0 operator console provides remote control of the system. On Aegis-equipped ships, the Mk 38 Mod 4 can also be integrated with the Aegis Combat System, allowing the weapon to use the ship's broader combat-system architecture for target engagement. This integration is one of the principal differences between the Mod 4 and preceding Mk 38 versions.

Counter-UAS and surface-defense role

The Navy developed the Mod 4 in response to shortcomings in earlier Mk 38 systems against unmanned aerial systems and unmanned surface threats. The 30 mm gun, improved fire-control chain, stabilized off-mount sensor and Aegis integration are intended to improve engagement accuracy, effective range and lethality. The weapon also retains surface-defense and anti-terrorism force-protection functions.

Manufacturer information identifies fast inshore attack craft and uncrewed aerial systems among the intended target sets. The Navy has also emphasized the system's ability to counter high-speed, maneuverable unmanned surface vehicles.

Operational history

USS Mustin was the first U.S. Navy ship to receive the Mk 38 Mod 4 and remained the publicly identified operational installation through 2026. The destroyer conducted live-fire training with the weapon in the Indian Ocean in August 2026.

The Navy intends to expand deployment beyond the initial ship. Planning cited in 2024 called for initial fielding on Arleigh Burke Flight IIA and Flight III destroyers. By February 2026, Dahlgren Division was finalizing the configuration for broader introduction as fleet requirements evolved.

Variants

• Mk 38 Mod 4: U.S. Navy configuration based on the Seahawk DS30M A2 family, armed with a 30 mm Mk 44 Bushmaster II and capable of carrying an optional coaxial 12.7 mm M2HB heavy machine gun. It uses the Mk 48 Mod 2 electro-optical sight system and Mk 134 Mod 0 operator console and can be integrated with Aegis.

Operators

• United States Navy: USS Mustin became the first U.S. Navy warship equipped with the Mk 38 Mod 4. Wider introduction is planned as the Navy finalizes the configuration for additional ships.

Specifications (Mk 38 Mod 4)

General characteristics

• Type: Remotely operated shipboard gun weapon system
• Gun mount: Mk 88 Mod 4
• Primary weapon: Mk 44 Bushmaster II 30 mm chain gun
• Secondary weapon: Optional coaxial 12.7 mm M2HB heavy machine gun
• Operator console: Mk 134 Mod 0
• Electro-optical system: Mk 48 Mod 2 EOSS
• Sensor equipment: Stabilized television and thermal cameras with laser rangefinder
• Combat-system integration: Aegis Combat System on compatible U.S. Navy ships

Performance

• 30 mm firing modes: Single shot, burst, low-rate automatic and high-rate automatic
• 30 mm automatic rate of fire: Approximately 100 or 200 rounds per minute
• 12.7 mm rate of fire: Approximately 500 rounds per minute

Armament capabilities

• 30 mm ammunition capability: Compatible with air-burst ammunition
• Primary roles: Counter-UAS, counter-unmanned surface vessel, defense against fast attack craft and close-range surface threats

Additional technical and procurement information is documented in the first-installation report.