The U.S. Marine Corps, responding to reports from their troops, are investigating whether they should replace the current 62 grain bullet, used in the 5.56mm round fired by the M-16 and M-4 rifle, with a heavier bullet. Since last Summer, the marines have been making the heavier, 77 grain bullet (normally only issued to Force Recon and commando troops) available to commanders, to use in place of the rounds with the 62 grain bullet. However, only six percent of the 10.6 million 5,56mm rounds of ammo the marines have in Iraq and Afghanistan, are the heavier 77 grain version. The debate over the effectiveness of the NATO standard 62 grain bullet (and all 5.56mm) ammo has been going on for decades, and has only intensified since 2002 (when it was used a lot in Afghanistan). The marines and the army are working together on the problem, and will present their findings early next year.
Many in the U.S. Army are in favor of using a larger caliber bullet (7.62mm, as used in sniper rifles like the M-14), or a 6.8mm round. The problem with the 5.56mm round was that it was not designed to take down man sized targets (or animal equivalents like white tailed deer, or black bears), and is less effective in blasting through walls and vehicles during urban fighting. When first introduced, it was intended for use by draftees, who were often in need of automatic fire capability (because so few were marksmen). This meant troops had to be able to carry more ammo, thus the utility of the 5.56mm round. The 5.56mm bullet could wound, or kill with a head or torso shot. But a determined enemy was often not stopped by 5.56mm fire. Today, all the infantry are volunteers, much better trained to hit targets with single shots, and increasingly demanding a bigger bullet for doing that.