Over the weekend, the Royal Navy released a statement about the SIG SAUER MCX LT carbines they procured under Project HAY, named after renowned WWII Royal Marine Ace fighter pilot Commander Ronnie Hay, DSO, DSC & Bar.

I’m going to share a few things I know about the program. The actual decision was made almost two years ago, with delivery well over a year ago. It is my understanding that the MCX carbines were purchased for maritime operations as part of the Royal Marines Future Commando Force operational concept with a renewed emphasis on amphibious raids and a fast-paced modernization of weapons and equipment.

The MCX carbines were procured chambered in 5.56 NATO and 300 BLK (7.62×35 for our European readers). However, it’s important to point out, how modular the system they purchased is. Due to the availability of upper receivers in 7.62×39, the Royal Marines could easily adapt if the need arises to add that caliber such as during operations with allied forces still using that caliber. Likewise, the Reduced Signature Assault Rifle (RSAR) configuration is also available and has been procured by USSOCOM as well as other friendly nations, in all three calibers.

As you can imagine, it can be a challenge operationally when you have one common platform and magazine that accepts two different cartridges. The Royal Marines have overcome this by purchasing 300 BLK carbines in all Black livery and 5.56 NATO chambered carbines featuring FDE accessories.

Every rifle comes fully equipped with suppressor, and optic (including magnifier).

The firm which served as the system integrator for the procurement, Edgar Brothers, has provided us with a statement which provides even more details:

Royal Marines Adopt SIG SAUER MCX Under Project HAY

The Royal Marines Commando Force has taken another significant step forward in its ongoing modernization with the adoption of the SIG SAUER MCX rifle under Project HAY, a program named after Ronald “Ronnie” Cuthbert Hay DSO, DSC and Bar, the only Royal Marines fighter ace. This follows the introduction of the Knights Armament Stoner 1 (KS-1) under Project COMACCHIO, which began replacing the SA80 as the primary weapon for the Royal Marines Littoral Response Groups under Project HUNTER (named in honor of Corporal Thomas Hunter, who was posthumously awarded the Victoria Cross for his bravery at Lake Comacchio). However, Project HAY focuses on specialized roles, replacing the venerable L119A1/A2 rifles produced by Colt Canada and ensures force-wide transition to AR controls.

The Legacy of Ronnie Hay

Ronnie Hay, a legendary fighter pilot and Royal Marine during World War II, epitomized the adaptability and daring that today’s Commando Force strives to embody. As the only Royal Marines fighter ace, Hay flew numerous combat missions in the Fleet Air Arm, earning multiple decorations for his aerial victories and ground-attack missions. His distinguished service included pivotal roles in the Dunkirk evacuation, the Battle of Britain, the Mediterranean campaign during Operation Torch, and the Far East, where he led one of the Fleet Air Arm’s largest operations and supported the assault on Okinawa. Hay’s remarkable legacy now inspires the cutting-edge capabilities delivered through Project HAY, as the Royal Marines continue to redefine their role as an advanced, multi-domain force.

Project HAY and the SIG SAUER MCX

The SIG SAUER MCX brings a level of versatility well-suited to the unique requirements of the Royal Marines’ maritime special operations-capable unit. This unit specializes in tasks such as ship interdiction, training partner forces overseas, and close-quarters engagements during boarding operations. Unlike Project COMACCHIO’s KS-1, designed for general issue, the MCX is tailored for specialized operations. Its design allows operators to switch between different configurations, making it highly adaptable to varying mission profiles. This capability is particularly valuable for training teams operating overseas, where non-standard ammunition may be the only viable option.

Compact and Mission-Ready

The shorter barrel lengths make the MCX ideal for confined spaces, such as ship interiors and urban environments. Both systems can be configured for suppressed or unsuppressed use, offering flexibility during sensitive operations where noise in confined spaces can produce challenges. It is understood that other variations made possible by the MCX’s flexibility are being considered and trialled. Other natures may be used that are more suitable for close-quarter battle or working with partner forces.

A further feature is the addition of RailScales’ custom Monomount, which raises optic height to improve compatibility with night vision goggles (NVGs). This feature enhances the weapon’s effectiveness in low-light environments while supporting ergonomic aiming. The Monomount also facilitates the top mounting of the LLM Mk3 laser, further enhancing the MCX’s versatility. Additionally, the MCX’s extensive stock options enable seamless integration with protective gear, including helmets with mandible attachments if required.

Training and Simulation

To ensure safe, realistic training, the MCX is equipped with dedicated Simunition kits for force-on-force exercises and blank-firing adapters for practice. These tools allow boarding teams to train under realistic conditions without compromising safety. The design of the MCX also ensures that operators can train extensively on one platform before transitioning to another for specific missions.

Lessons Learned from the HUNTER Programme

The Royal Marines’ evaluation of the MCX benefited greatly from Project HUNTER, which enabled extensive small arms experimentation. This initiative allowed units to field-test systems and provide valuable feedback on their performance. For Project HAY, the results of these trials were instrumental in shaping the final configurations of the fielded MCX.

A Purpose-Built Tool for the Future Force

The adoption of the SIG SAUER MCX marks another milestone in the Royal Marines’ transformation into a special operations-capable maritime task group. The MCX, with its design and adaptability, gives the Commando Force a purpose-built tool for modern operations. Whether conducting training missions in remote regions or executing high-stakes maritime interdictions, the weapon’s compact size, suppressor compatibility, and flexibility make it an invaluable asset for deployed teams.

Mike Newman, Director PMD at Edgar Brothers, commented on the collaboration: “It was a great effort between ourselves, UK Commando Force, and SIG SAUER to provide the MCX and associated ancillaries. Edgar Brothers are hugely proud to provide another weapon system into the Royal Marines arsenal.”

Ron Cohen, President and CEO of SIG SAUER, Inc., added: “SIG SAUER is proud that our rifles, suppressors, and optics are entering service with the UK Royal Marines. This system of cutting-edge products is designed for proven performance and reliability to withstand the rigors of the battlefield. We are honored to support the Royal Marines with our products and their commitment to mission readiness.”

Under Project HAY, the Royal Marines continue to honor the legacy of Ronnie Hay by equipping specialist units with the tools to thrive in all domains, just like Ronnie Hay.