General Dynamics initiated development of the LWMMG in-house based on reports of combat experiences from U.S. soldiers and Marines in the mountains of Afghanistan around 2010. These reports identified that a particular issue was that with combat often commencing at ranges of 1,000 meters (~1,100 yards) or more, insurgents armed with PKMs were difficult to engage with the M240, as at extreme ranges, the 7.62mm NATO cartridge has noticeably more drift than the 7.62×54mmR. The stated goal of the project was to develop a weapon with the range of a heavy machine gun but the portability of a GPMG. The weapon was first displayed on May 15th, 2012 at the Joint Armaments Conference in Seattle, Washington. An improved model with the option for a suppressed barrel is unveiled in 2014 at AUSA with the weight of the weapon reduced to 22 lbs (10 kg).
The concept of a machine gun firing a round more powerful than a standard rifle caliber but outside the modern definition of a heavy machine gun is not a new one: there was a brief trend for for such weapons in the 1930s, resulting in what could be termed "heavy medium" machine guns chambered for calibers such as 8×59mmRB Breda.
In May of 2017, the United States Marine Corps and SOCOM issued a Sources Sought Solicitation for 5,000 machine guns chambered for a polymer-cased .338 Norma Magnum cartridge, weighing 24 pounds or less with a 24-inch quick-change barrel with the option of a suppressed barrel, compatible with the existing M192 tripod and fitted with a lightweight bipod, with a rate of fire of 500-600 rounds per minute and a range against area targets of no less than 2,000 yards. Since the LWMMG is the only machine gun that is currently chambered for .338 Norma Magnum and these specifications are obviously based on those of the weapon, it is likely that it will be adopted, with sources claiming that it is intended to supplement the Browning M2 (as it cannot match .50 BMG's effectiveness against light armored vehicles or fortifications) and replace the M240 in these services.
The LWMMG is built around a modified version of the "differential firing" mechanism that General Dynamics had previously developed for the XM312, XM307 ACSW and XM806 weapons. This new variation is referred to as "short recoil impulse averaging." Like the prior weapons (as well as several others, such as the G11 and AN-94) the LWMMG's internals are a distinct sub-receiver inside the main one, with the operating parts of the weapon all being capable of motion inside the static outer receiver. The LWMMG uses this system in a method analogous to API blowback, releasing the mechanism from the rearward position and then firing while it is still moving forward.
The recoil reduction offered by this system is said to result in felt recoil similar to an M240, despite the gun being the same weight as the M240L and almost six pounds lighter than the M240B, while firing a much more powerful round. It seems that General Dynamics has finally solved the rate-of-fire issues that had doomed previous iterations of their recoil-reduction technology, as the LWMMG fires at 500 rounds per minute as opposed to the 260 and 265 of the XM312 and XM806, respectively.
The complete system was quoted as weighing in at 105 pounds (47.6 kilograms) for the first variant: this comprises the weapon, an M192 machine gun tripod, 500 rounds of ammunition, a spare quick-change barrel and the supplied ACOG optic.
The LWMMG is designed specifically with replacing the M240 in mind and requires no alteration to existing M240 mountings in order to fit on them.
The LWMMG fires the .338 Norma Magnum (8.6×63 mm), a Swedish precision rifle cartridge originally designed by American sport shooter Jimmie Sloan with the help of Dave Kiff of Pacific Tool and Gauge. It fires a 300 grain (19.44 gram) projectile with a muzzle energy of over 4,600 foot-pounds (6,300 joules). This makes it four times more powerful than a standard 7.62mm NATO round at 1,000 yards (though on the downside, both the rounds and belt links are twice as heavy), and more accurate at range than both 7.62mm NATO and .50 BMG. The weapon can be converted to fire .338 Lapua Magnum if necessary.