The recent decision, by the U.S. Marine Corps, to replace many of its 5.56mm M249 light machine-guns with a lighter and more compact 5.56mm weapon (the M27 automatic rifle) reflects 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 Germans also developed the first automatic assault weapon. This was the 9mm MP 18, a "submachinegun" that would eventually evolve into the modern assault rifle. By the end of World War I, about 30,000 MP 18s were in use. The MP 18 demonstrated the devastating effect of automatic weapons in the hands of most members of an infantry unit. The MP 18 fired the standard 9mm pistol round and used a 32 round drum magazine that fired 6-7 bullets a second. The basic need was for a compact weapon that could quickly fire a lot of bullets. This gave the MP18 user a big edge in combat. The Germans kept developing this type of weapon and by World War II they had the MP 38 and MP 40. The short range (50-100 meters) of the 9mm pistol round prevented the Germans from attempting to rearm all their infantry with this weapon, who often had to hit targets farther away.
There was still a need for a longer range automatic weapon, and in the 1960s Russia and the United States developed weapons using more powerful rounds (the ones used in the AK-47 or M-16) in light machine-guns. These were OK for the anticipated Cold War battles, but in the last decade, American troops have largely been conducting raids in urban areas. These put a premium on compact automatic weapons, and the M249 was not compact.
A similar evolution took place during World War II, although informally. When there was a lot of urban, or close quarters, fighting, as during the battle for France in 1944, the subsequent invasion of Germany, and some of the battles in the Pacific, allied troops would improvise. More portable automatic weapons would be scrounged up, and the official squad organization ignored. Some existing machine-guns were made portable, by leaving the tripod behind and modifying the weapon so it could be fired while advancing on foot.
The new marine M27 also reflects that combat has been changed by a lot of new technology. The increased use of snipers (and better sniper rifles) has provided precision long range fire. Micro-UAVs like the two kilogram Raven give unprecedented views of the battlefield, and GPS guided bombs and projectiles enable the troops to get closer to a fortified enemy, and less need for long range machine-guns.
For the moment, the marines are hanging on to most of their M249s, but are using the M27 to better fight the new type of infantry combat that has developed over the last decade.