For the last four years, the U.S. Marine Corps has been seeking a replacement for its 5.56mm M249 light machine-guns. While the winner of the competition was the 5.56mm M27 (a variant of the H&K 416), this was more because the marines decided they preferred an automatic rifle, rather than a better LMG (light machine-gun). But one of the competitors, the 5.56mm FN HAMR (Heat Adaptive Modular Rifle), was very impressive as a better LMG. The big innovation in HAMR is the use of a special heat sensitive material in the firing chamber that changes from a liquid to a solid as the chamber (where the shell goes before the propellant is ignited and the bullet fired) gets so hot that it can ignite the propellant and fire the round without the user pulling the trigger. This overheating and "cook off" of the round is not good, especially with a machine-gun. When the heat sensitive material goes solid, it forces the HAMR to fire from the open-bolt position. That means that the bolt (that pushes the round into the firing chamber and uses its firing pin to ignite the propellant when the trigger is pulled) is locked in the open position whenever the weapon is not firing, allowing the chamber to cool off much faster. The open-bolt status remains until the chamber has cooled off enough for the heat sensitive material to return to its liquid form, at which point the HAMR returns to the more efficient closed bolt position (where you get your first shot off faster and with more accuracy). The heat sensitive system works automatically, although the user will note the open bolt mode, which indicates that the weapon is getting hot, and maybe the amount of firing done should be reduced for a minute or two to help cool off the chamber and barrel. Depending on the air temperature, HAMR has to fire 400-500 rounds quickly, on full automatic, before the open bolt mode kicks in.
HAMR is based on the FN SCAR assault rifle, which was designed for U.S. SOCOM (Special Operations Command), so it is a mature basic design. HAMR is also designed to produce less recoil, and has a slower ROF (Rate of Fire) of 625 rounds per minute (versus up to 1,000 for the current M249). But because HAMR is so much more accurate, you get more hits with the lower rate of fire, meaning you have to haul around less ammo to do the same amount of damage.
HAMR is a lighter weapon (5.1 kg/11.2 pounds) that looks like an assault rifle (which is nice, because anything that looks like a machine-gun tends to attract more enemy fire.) HAMR can use regular 20 or 30 round assault rifle magazines, or 50 or 100 round box magazines. An overheated barrel can be quickly changed. A belt fed version is in the works.
The marines adopted the lighter and more compact 5.56mm weapon (the M27 automatic rifle) to reflect a shift in tactics, as well as weapons. The M27 is a 84 cm (33 inches) long, 3.6 kg (7.9 pound, empty) automatic weapon that has a forward grip and heavier barrel. It can use a standard 30 or special 100 round magazine, but the barrel cannot be changed (when it becomes overheated from firing too many rounds). The M249 is 104cm (41 inches) long, weighs 6.8 kg (15 pounds) empty and uses belted ammo. The barrel can be changed when it overheats.
The M249 was developed to give the infantry squad more firepower. Army squads have two of them, marine squads had three, on paper, but often went into action with only two, with the third machine-gunner carrying extra ammo and machine-gun barrels. The M249 was a classic machine-gun, designed to put out a lot of bullets. A century ago, the Germans concluded that the machine-gun was the "essence of infantry" and by the end of World War I had created the modern infantry squad. This was a small unit (about ten troops) equipped with a light (portable) machine-gun that was supported by the other members of the squad (who carried additional machine-gun ammo and protected the machine-gun operator.)
But the marines have hedged their bets. They have bought 4,200 of the new M27s, which will replace the current M249s in marine infantry battalions. However, each of these battalions will retain six M249s, to give the battalion some options. The marines will withdraw from service 20 percent of their 10,000 M249s by the time all 4,200 M27s have been delivered. But the marines may still decide to replace the remaining M249s with HAMR. The U.S. Army is also looking into a M249 replacement, and HAMR is a contender there as well.