Well, the Marine Corps is going retro. In World War I, the Germans realized that the tripod mounted water-cooled machine gun was not flexible enough for combat that moved fast. They worked on the sub-machinegun and on lighter machine guns, but not until 1935 did they get the idea of a general purpose machine gun out to the troops, the MG-34. Much lighter than water cooled or other exclusively tripod mounted machineguns, it usually deployed from a bipod and gave the infantry squad greater firepower. The system was perfected with the later MG-42. The Germans also invented the assault rifle. Both systems were universally adopted by all the world's armies sooner or later.
During WWII, most of Germany's opponents went half measures, they used magazine fed squad automatic rifles, like the M-1918 (BAR) in the U.S., the Lewis Gun, and the Bren Gun. While used by U.S. and British forces successfully, their shortcomings were obvious. Their magazines restricted their volume of fire and was a great disadvantage when in combat with the German belt fed light machineguns.
All the magazine fed squad automatic rifles were enventually replaced first by the M-60 light machine gun in U.S. forces and the MAG in British forces, then the M-60 was replaced by the M-240, which was also supplemented by the smaller caliber M-249 and variants. All were basically the same German belt-fed light machinegun usually deploying from a bipod.
But, were are going back to the future: The USMC and the Army are looking to replace the M-249 and variants with, the modern equivalent of the BAR or Bren. http://www.military.com/news/article/corps-seeks-replacement-for-m249-saw.html
We will see how this turns out, but the Marines are looking to use a variant of the standard M-16 assault rifle, which has shown its problems in Iraq. It seems that everyone in Iraq that won a medal for combat, including the lost little girl Jessica Lynch's famous battle involved some sort of battle failure of the M-16 varients. Do we really want to go back to the future and continue with a weapon system with reliability problems? Although I will say although the BAR and Bren had issues, mostly weight and conplexity, reliability was not one of them. Hell, why not go to an obscure version, the Johnson light machinegun? At least our Marines and soldiers will have a weapon chambered for a more lethal cartridge.
But the question is: If the M-249 and M-240 are too heavy, why not improve them? Or replace both the the improved version of the M-60; the M-60A4, an improved version of the previous M-60 varieants, lighter in weight and more dependable. Let's not move backward. Guns should be like Camrys, same model, but improved incrementally each year.