U.S. Navy Restores USS Donald Cook’s 5-Inch Gun After 2-Year Failure While Supporting Iran Operations.

The U.S. Navy has restored the Mk 45 5-inch gun aboard the Arleigh Burke-class guided-missile destroyer USS Donald Cook (DDG-75) after a persistent casualty kept the weapon out of service for two years. The repair returned the destroyer’s 127 mm gun to full naval-gunfire-support readiness while the ship was deployed in support of ongoing operations involving Iran, the U.S. Navy said.

The Mid-Atlantic Regional Maintenance Center disclosed the repair on October 1, 2026, after Navy technicians traced the malfunction to a failed Fire Control Interface Circuit Card that prevented a required 25-volt direct-current signal from reaching the gun system. A subsequent live-fire exercise produced what the Navy described as highly satisfactory results, verifying the repair under operational conditions.

Related Topic: U.S. F/A-18F ‘Sub Buster’ Marking Points to Possible Iranian Kilo-Class Submarine Kill in Operation Epic Fury

The U.S. Navy USS Donald Cook (DDG-75) sails alongside USS George H.W. Bush (CVN-77) on June 18, 2026. Although the destroyer appears fully operational externally, the U.S. Navy later disclosed that a two-year internal fire-control casualty affected its Mk 45 5-inch gun before it was restored to service. (Picture source: U.S. Department of War/Defense)

The U.S. Navy USS Donald Cook carries the Mk 45 Mod 2 5-inch/54-caliber gun used aboard early Flight I and Flight II Arleigh Burke-class destroyers. The weapon can engage surface and land targets and has a conventional-ammunition range of about 13 nautical miles, or 24 kilometers, with a maximum automatic firing rate of approximately 16 to 20 rounds per minute. Restoring the gun therefore gives the destroyer back an important layer of rapid-firing conventional naval firepower for surface engagements and naval surface fire support.

The casualty did not make Donald Cook generally non-combat-capable during the two-year period. The destroyer retained its Mk 41 Vertical Launching System for weapons including Standard Missiles, Tomahawk cruise missiles and anti-submarine rockets, as well as Phalanx close-in weapon systems, torpedoes and other shipboard armament. The Mk 45 failure nevertheless removed a distinct capability from the ship’s weapons inventory for two years.

According to the U.S. Navy account, previous troubleshooting efforts repeatedly focused on components identified in standard technical documentation without correcting the malfunction, forcing the latest repair team to look beyond the normal diagnostic sequence. The breakthrough came when technicians Brian Hall and Adam Harlan of MARMC’s Surface Fire Control/Guns Branch disabled the compensation circuit inside the Electronic Servo Control Unit. The gun immediately returned to its commanded position, allowing the technicians to follow the missing electrical signal back to the defective Fire Control Interface Circuit Card.

Getting those specialists to the deployed destroyer required an unusually complex logistics operation. Hall and Harlan traveled from Norfolk through Europe and Djibouti before flying roughly seven hours aboard a CMV-22B Osprey to an aircraft carrier in the region, with the tiltrotor aircraft receiving three aerial refuelings during the flight. They were then transferred onward by MH-60R Seahawk after heavy seas complicated an earlier attempt to reach Donald Cook.

The movement illustrates the logistical demands involved in delivering specialized maintenance expertise to a deployed surface combatant operating far from its home maintenance infrastructure. Repair work continued while the ship remained underway, with technicians working around the destroyer’s operational schedule and weather conditions that periodically restricted access to exposed equipment because sea spray risked damaging electronics.

Once the U.S. Navy identified the failed circuit card, it faced a second logistical challenge: getting a replacement component to a warship operating in the Indian Ocean. MARMC personnel in Norfolk expedited the card commercially to Singapore, where Donald Cook’s supply personnel arranged onward movement through Diego Garcia. The component reached the destroyer in 12 days, significantly compressing a supply process that MARMC said can normally take more than a month for an underway ship in the region.

After installing the card and resolving secondary electrical faults, Donald Cook’s crew conducted the live-fire verification that returned the Mk 45 to service. The event demonstrates that forward-deployed combat readiness depends not only on carrying advanced weapons but also on the ability to diagnose electronic failures and move specialized personnel and low-volume components across intercontinental supply chains.

In Donald Cook’s case, a single failed circuit card had removed the destroyer’s principal medium-caliber naval gun from its available weapons for two years. Its restoration is particularly relevant during an active deployment that the U.S. Navy says supports ongoing operations involving Iran, where surface combatants may need to maintain several layers of offensive and defensive capability simultaneously.

The Mk 45 does not replace Donald Cook’s missile-based strike or air-defense systems, but restoring it expands the commander’s available options for surface warfare and naval gunfire missions and returns an important conventional weapon to the destroyer’s combat inventory. The U.S. Navy’s live-fire event should be understood as a post-repair operational verification rather than evidence that Donald Cook has used the Mk 45 against an enemy target. The firing confirmed that a long-running technical casualty has been cleared and that the destroyer has regained a weapon capability that can again contribute to its broader multi-mission combat loadout during deployment.

Explore More Defense News

• Land Defense News
• Naval Defense News
• Defense Aerospace News

Written by Alain Servaes – Chief Editor, Army Recognition Group
Alain Servaes is a former infantry non-commissioned officer and the founder of Army Recognition. With over 20 years in defense journalism, he provides expert analysis on military equipment, NATO operations, and the global defense industry.