The Ultimate Firearms Destination for the Gun Lifestyle


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In this partnered content series, RECOIL explores the SIG Sauer Defense Strategy Group’s (DSG) efforts to develop next generation small arms solutions for the U.S. military. Our first story in this series covers SIG’s effort to develop its first machine gun that ended up producing both the SIG-MMG 338, developed as a submission for U.S. SOCOM’s ongoing LMG-M program, and the SIG-LMG 6.8, which the U.S. Army’s chose as its Next Generation Squad Weapon – Automatic Rifle (XM250).

In 2017, U.S. Special Operations Command (SOCOM) challenged industry to provide potential solutions that could fill the gap between its current man-portable, medium machine gun—the 7.62 NATO M240B/L, capable of effectively engaging targets up to 800 meters away, and its 84-plus-pound sibling—the 50 BMG M2A, that reaches past 1,800 meters.

The gap between these two machine guns had left the command’s maneuver units unable to provide standoff or deal with threats between 800 and 1,800 meters without relying on the stationary or vehicle mounted M2A. SOCOM needed a man-portable machine gun that operators could carry and use to assault and defend against distant targets as they took and held ground.


The concept of a lightweight medium machine gun began to take shape during the war in Afghanistan. American units tasked with holding ground in remote, sometimes bowl-shaped terrain were attacked or harassed by infiltrating enemies who came over the top of mountains surrounding their positions. The effective enemy plunging fire was difficult to respond to with the relatively short-ranged 7.62 NATO round of the M240.

A 2005 view of Combat Outpost Keating on the Pakistan-Afghanistan border in a remote pocket of Afghanistan known as Nuristan. Soldiers stationed here said being at COP Keating was like being in a fishbowl or fighting from the bottom of a paper cup. (U.S. Army photo by 1st Lt. Brad Larson/Released)

This tactic is illustrated in the 2009 battle of COP Keating, where the outpost was overrun, and eight soldiers and four Afghan allies were killed by Taliban fighters attacking from the high, steep mountainsides surrounding the base. The enemy recognized that the American’s best means of fighting back (at least until air support could come on station) was with their longer ranged .50 caliber M2A machine guns. The attackers massed fire on the emplaced guns and gun trucks, taking them out of the fight in the early stages of their attack.


By 2016, the problematic capability gap between America’s medium and heavy machine guns was well defined and the solution was identified in the form of the 338 Norma Magnum (338 NM) caliber cartridge. The 338 NM cartridge provides triple the ballistic performance of a 7.62 NATO cartridge fired from an M240B. Its performance is closer to that of the 50 BMG, but at less than half the weight and size of the .50 cal. round, and when carrying a machine gun, weight is paramount.

In 2017, with the procurement process only in the ideation phase, SOCOM hosted an industry day and invited interested vendors to submit their ideas and designs for a lightweight medium machine gun (SOCOM used the nomenclature “LWMMG” at the time). Instead of stating hard requirements for the vendors’ systems to meet, SOCOM offered general performance goals and capabilities in an effort to give industry wide latitude in its response.

However, SOCOM did have a few sticking points that were non-negotiable including weight, dimensions, and caliber, which provided the vendors a hint of the weapon it thought would fit its needs.

What they were asking for was a sub-24-pound, belt-fed, 338 NM-chambered machine gun with suppressed and unsuppressed, quick-change, 24-inch barrels, a rate of fire from 500 to 600 rounds per minute (RPM), and an area target range of 2,000 meters.

The challenge piqued the interest of the SIG Sauer defense team and engineers, and thus the SIG Machine Gun program came into existence; the program, in its infancy, was founded with the mission to fill the U.S. machine gun capability gap with a SIG-designed and built machine gun chambered in 338 NM. Ron Cohen, CEO, had always envisioned that SIG Sauer would evolve to become the premier provider of small arms to the U.S. military with the crown jewel of that suite of weapons being a machine gun. This was that opportunity.

The company assessed its capabilities, marshaled its forces, and assembled a dedicated machine gun design team that would eventually number 25 members within its engineering efforts and Special Weapons Group (SWG).


Between 2017 and 2018, SIG developed its very first machine gun, a lightweight medium machine gun prototype called the SIG-Medium Machine Gun 338 (SIG-MMG 338).

The traditional approach in building a lightweight machine gun is to build a reliably working prototype, then shave weight wherever possible in ways that the gun is still able to meet its performance and reliability objectives. The challenge with this approach is that the lightest version of the gun remains elusive, as engineers, by practice, tend to be cautious and will leave meat on the bone for fear of compromising reliability.

The SIG Sauer Defense Strategies Group began work on its SIG-MMG 338 in 2017. (SIG Sauer)

“While this method would produce a light gun,” says SIG Defense Strategy Group’s (DSG) Senior Director of Strategic Defense Products, Jason St. John, “it won’t produce the lightest gun and the directive was to design the lightest gun, so to be successful we had to buck convention, which quite honestly is what we do it SIG.”

To achieve this, SIG’s engineers worked in reverse. They focused on building the lightest operational gun they could and testing it to discover the points of failure. Failed parts were redesigned and when material was added, it was done so strategically to ensure the weapon remained as light as possible.

In this early engineering stage, each redesigned component was developed nearly independently of the other parts around it. Then, with each of the improved parts combined into a working prototype, the “Franken-gun” was tested, and its parts were further tweaked for reliability.

This process played out over a couple of iterations until the first prototype SIG-MMG 338) was completed in 2018. This first prototype produced the absolute lightest functioning medium machine gun that weighed under 21 pounds with a 50-85 mean rounds between stoppages (MRBS). While the weight was ideal, the MRBS still had a way to go before it was ready for combat.


At this point, there was still no contract issued or even a hard-lined requirement statement from SOCOM. The SIG-MMG 338 program remained an elusive vision, but that didn’t matter to SIG’s Cohen. The decision to invest significant resources in R&D, personnel, and facilities to support the program without a clear design goal, or, let alone, a firmly committed customer, was not about the contract, it was bigger than that, it was about ensuring future capabilities for our warfighters.

But, in late-2017 a very interesting opportunity revealed itself that would change military small arms forever. While SIG was in the middle of developing its first SIG-MMG 338 prototype, the U.S. Army issued the requirements for its Next Generation Squad Weapon (NGSW) submissions.

The decision for SIG to compete in the NGSW program was instantaneous, and with the resources already in place to develop the SIG-MMG 338, SIG was poised to deliver. The company’s machine gun program had a new mission, and the resources were shifted from the quickly maturing SIG-MMG 338 program to the effort to build a small caliber machine gun for the Army program.

While this meant that progress on the SIG-MMG 338 would temporarily slow, all the progress and development from this point gave the company a head start in the development of the SIG Lightweight Machine Gun 6.8 (SIG-LMG 6.8), that would compete for years against the industry giants in this category, and ultimately prevail to become the NGSW XM250, the official replacement for the U.S. Army’s 5.56 NATO-chambered M249 SAW.

SIG Sauer personnel developing and testing the SIG-LMG 6.8 for the U.S. Army’s XM250 NGSW program at its Advanced Weapon Research Facility in Epping, NH. (SIG Sauer)


SIG Sauer was now working on two machine gun programs at once. The majority of its machine gun development effort was temporarily shifted from its work on the SIG-MMG 338 and was now directed toward the U.S. Army’s NGSW XM250 program.

While this slowed development of the large caliber machine gun, the decision to split the effort would pay dividends later, as lessons learned while working on the light machine gun would later be applied to the medium machine gun.

Now working on the second-generation prototype, SIG’s engineers put significant effort into SOCOM’s caliber conversion capability requirement. The command required its lightweight medium machine gun (LWMMG) to run 7.62 NATO ammo in addition to 338 NM for both operational use and as a low-cost training option.

While it seems counterintuitive, running the large caliber machine gun operating system with a smaller cartridge is actually harder on the system and produces more wear. This presented a difficult engineering problem for SIG’s engineers to overcome, and we’ll discuss this issue, and more, in an upcoming story detailing the technical development of the SIG-MMG 338.

The (roughly) year of work by the SIG-MMG 338 team resulted in an improved, second-generation (Gen2) prototype that was finished in early 2019. At 23 pounds, it weighed slightly less than its Gen1 predecessor, and, just as important, its operational reliability jumped up to 300-450 MRBS after incorporating lessons learned from the company’s work on the SIG-LMG 6.8 program.

Crucially, in 2019, the company’s initial gamble to embark on SOCOM’s LWMMG project showed some real promise. That was when the command held a vendor range day and assessed three competing lightweight medium machine gun submissions from three vendors. After evaluating each offering, SOCOM bought 10 of SIG’s second-generation SIG-MMG 338s.

The command would use to evaluate and develop its capability and performance requirements for the LWMMG program. Before it was put to use by the military, though, the Gen2 SIG-MMG 338 had to be safety certified by the military. That rigorous testing was performed by Naval Surface Warfare Center-Crane Division, which certified the gun safe for use in SOCOM’s combat evaluations. This validation was quite an accomplishment for a company that had only made its first machine gun a year prior.


In 2020, as work on the third-generation (Gen3) MMG 338 prototype progressed, SOCOM released its first draft request for prototype proposals (RFPP) that included initial programmatic requirements for its renamed Lightweight Machine Gun-Medium (LMG-M) program.

This draft gave industry a more defined set of design and performance goals, and, perhaps more importantly to those companies, like SIG, that were leaning forward and spending millions on their development efforts, it indicated SOCOM’s solid intention to procure a lightweight medium machine gun.

It would take two years to develop a third prototype SIG-MMG 338, as machine gun design resources previously diverted to the SIG-LMG 6.8 program were eventually returned to the larger caliber program. The company’s decision to shift its efforts to the LMG 6.8 program would pay off in grand fashion, though.

SIG-LMG 6.8 developments were now feeding back into the SIG-MMG 338 program, boosting the gun’s reliability, overall performance, and even providing parts commonality in some areas. Development also benefited from SOCOM’s evaluation of the second-generation gun, since SIG’s engineers were able to incorporate user feedback in its third development cycle.

Compared to the second-generation SIG-MMG 338, the Gen3 incorporated a completely redesigned receiver and handguard, a vastly more durable operating group, an improved link stripping mechanism, a simplified feed tray cover latch, a purpose-built machine gun buttstock (made by SIG), a new buttstock hinge, and an updated quick-change barrel handle.

But, most importantly, drastic reliability and performance improvements were realized at this stage resulting in a third-generation gun weighing 24.2 pounds that approached true machine gun reliability, achieving 2500 MRBS during testing in early 2021.

SIG Sauer went through four generations of research and development spanning about seven years to bring the SIG-MMG 338 to life. The prototypes from Gen1, rear, to Gen4, front. (Rob Curtis/RECOIL)


At this point, in 2023, SOCOM issued an Other Transaction Agreement (OTA) to industry for the LMG-M, which better reflects the program’s effort to develop new technology, as opposed to the use of a traditional Federal Acquisition Regulation (FAR)-based indefinite duration indefinite quantity (IDIQ) contracting vehicle that’s less flexible and more geared toward procuring commercially available products. The OTA largely reflected the requirements dictated in the prior draft RFPP and provided vendors a late-2023 deadline to submit samples.

To meet the OTA deadline, SIG had about six months to incorporate further refinements into a fourth-generation SIG-MMG 338. These refinements resulted in a 25.34-pound, fully operational machine gun that provided greater than 3500 MRBS. The update included improvements to the trigger pack (based on a design shared with the SIG-LMG 6.8/NGSW XM250), handguard, barrel swap system, and other subtle changes that reduced dispersion while increasing reliability and durability.

SIG met the deadline, although the ultimate submission date was extended to early 2024, which provided the company more development time to further maximize its submission.

SIG’s LMG-M submission was accepted by SOCOM, which is in the process of safety certifying it, alongside two competing submissions from other firms. The LMG-M submissions will all undergo a down-select process that includes both the Operator Test/Military User Assessment and full spectrum testing.

Based on its evaluation and down-selection of the OTA submissions, SOCOM is expected to announce a formal LMG-M request for proposal (RFP) by summer 2024. Once that RFP is released, it will give the down-selected vendor the ability to refine its submission to meet any adjustments to the program’s requirements SOCOM may have made following the down selection process. SOCOM will award an IDIQ contract to the down-selected vendor shortly after.


The SIG-MMG 338 is capable of not only being converted to standard 7.62 x 51mm to align with NATO ammunition and current stockpiles, and it’s specifically designed to handle high-pressure ammunition such as the Army’s new hybrid 6.8 x 51mm NGSW ammunition and is poised for the use of a hybrid 338 NM cartridge, should it be adopted.

The effort to design the SIG-MMG 338 was the result of more than a million rounds of testing over approximately seven years of development. In that time, SIG invested millions in personnel and capital, including the construction of a dedicated machine gun testing facility, to turn an idea into a reality in the form of a fully operational lightweight medium machine gun.

The SIG-MMG 338 is one of three top contenders to be adopted by SOCOM, inarguably the world’s most advanced fighting force. According to SIG, it’s also among machine guns that 13 other nations are currently considering adopting for their militaries.

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