The guided-missile destroyer USS Winston S. Churchill (DDG 81) fires a Mk 45 Mod 4 5-inch 62 calibre lightweight gun during a live-fire demonstration in the Atlantic Ocean, 19 April 2021. Illustrative of the mount type discussed; not connected to the Diehl–BAE Systems agreement. U.S. Navy photo by Mass Communication Specialist 3rd Class Katie Cox, via DVIDS (public domain).

Diehl and BAE Systems Move to Marry Guided 127 mm Ammunition to the Mk 45 Mod 4 Naval Gun

Diehl Defence and BAE Systems announced on 3 August 2026 that they will integrate Diehl's long-range 127 mm ammunition family with the BAE Systems Mk 45 Mod 4 naval gun, under a memorandum of understanding aimed at the German and international markets. A second strand of the agreement studies fitting the IRIS-T missile family to BAE Systems launchers.

Technical Summary

The Mk 45 Mod 4 is a 127 mm, 62 calibre naval gun mount produced by BAE Systems and fitted to surface combatants across several NATO and partner navies. Its 62 calibre barrel, longer than the 54 calibre barrel of earlier Mk 45 variants, was introduced to accommodate the higher chamber pressures and longer propelling charges that extended range natures require. That design margin is what makes the mount a credible host for guided projectiles rather than conventional high explosive rounds alone. BAE Systems is separately upgrading in service United States Navy mounts to the Mod 4 configuration.

Neither company named the specific 127 mm natures in scope. Diehl Defence's publicly known long range 127 mm offering is the Vulcano family, a sub calibre guided projectile developed with Leonardo and supplied in ballistic extended range and guided long range configurations. Leonardo's published product data puts the ballistic extended range nature at up to 60 km and the guided long range nature at up to 100 km with inertial and satellite guidance, quoting a circular error probable of 5 m or better and offering a semi active laser or infrared terminal sensor as an option. Set against the conventional ballistic natures the mount was built to fire, quoted by the United States Navy at 13 nautical miles or about 24 km, that is the scale of the capability being discussed. Manufacturer range claims are quoted here as published and are not independently verified by ISC. The German Navy completed operational suitability firings of Vulcano 127 mm from the F125 class in November 2022, though on a Leonardo 127/64 lightweight mount rather than a Mk 45. Diehl also took a NATO Support and Procurement Agency contract for Vulcano 155 mm guided ammunition on 7 July 2026. The absence of a named nature in the 3 August announcement is recorded here as a data gap, not treated as confirmation that Vulcano is the round concerned.

The IRIS-T strand has a precedent. Diehl Defence signed a comparable memorandum with Lockheed Martin at the Berlin Security Conference on 18 November 2025 to study integrating the IRIS-T family into the Aegis combat system and the Mk 41 and Mk 70 vertical launching systems. A second launcher agreement with BAE Systems the following August suggests Diehl is pursuing parallel integration routes rather than committing to one prime, which is a sensible hedge while the German F127 programme settles its combat system architecture.

The Mk 45 Mod 4 mount carries a 62 calibre barrel against the 54 calibre barrel of earlier variants, an eight calibre increase that exists to accommodate the higher chamber pressures and longer propelling charges extended range natures demand. ISC technical assessment, 4 August 2026

Analysis of Effects

Marrying a guided projectile to an existing gun mount is not a matter of chambering alone. A gun launched guided nature has to survive setback acceleration at launch, the spin imparted by the rifling and base pressure loading, all while carrying a guidance section, control actuators and a power source. Open literature on 127 mm and 155 mm guided projectiles places setback loads in the order of ten thousand times gravity. The qualification level to which the Diehl 127 mm family is proved is not disclosed, so the engineering margin available for integration cannot be assessed from open sources.

The second constraint is the mount itself. The Mk 45 is an automated system with a below deck loader drum and hoist, so any new nature must match the length, mass and centre of gravity the handling machinery was designed around, and the fuze setter must be able to programme the projectile before it enters the breech. A sub calibre guided round carried in a discarding sabot is dimensionally awkward in exactly this respect. Integration work of the kind this memorandum describes is therefore best read as mechanical and interface qualification rather than a marketing exercise.

The ready service drum sets a hard number on that trade. Published figures for the Mod 4 give the drum a capacity of 20 ballistic rounds or 10 extended range rounds, with the two able to be mixed in the same load. Extended length natures are programmed through a setter hood that fits over the projectile nose and transfers target and navigation data inductively, quoted at around 2.6 seconds per round. Any guided 127 mm nature therefore has to be qualified against three separate interfaces, the drum and hoist geometry, the setter hood, and the rammer, and if it falls in the extended length category it halves the rounds available at the mount before a resupply cycle. Whether the Diehl natures in scope sit in the ballistic or the extended length class is not stated, and the loading dimensions of the family have not been published, so the effect on ready service capacity cannot be quantified from open sources.

Personnel and Safety Considerations

For magazine planning the change is material. A conventional 127 mm high explosive projectile with fuze fitted is normally assigned to Hazard Division 1.2, Compatibility Group E, with its cased propelling charge held as Hazard Division 1.3, Compatibility Group C. A guided nature trades explosive fill for guidance section volume, so its net explosive quantity per round is lower than the conventional equivalent, while the round introduces a lithium power source and electro explosive devices that can move it into a different compatibility group. Neither the hazard classification nor the net explosive quantity of the Diehl 127 mm natures has been published, so ship magazine quantity distance and stowage segregation cannot be assessed from open sources. Any operator taking the round to sea will need the classification confirmed against AASTP-1 Edition C and its national ammunition safety authority before a stowage case can be written.

Data Gaps

The specific 127 mm natures covered by the agreement are not named. No hazard division, compatibility group or net explosive quantity is published for the Diehl 127 mm family. No contract value, customer or in service date has been announced, and the agreement is described only as a memorandum of understanding covering technical integration, which commits neither party to production. The German requirement said to be driving the work is not identified. Whether a guided nature fits the existing Mk 45 loader drum and hoist without modification is unstated, as is whether qualification firings have taken place.

Corrections & updates welcome. If you hold open-source data that refines or corrects any parameter in this article, please contact [email protected] citing the specific claim and your source. Verified corrections will be incorporated and credited in the revision history. AI-assisted technical assessment based on open-source material. Not a formal intelligence product.