Wars are about killing people, and the more quickly and efficiently you kill the other guy the more likely you are to win. But there are international restrictions, generally referred to as the rules of war, on the types of weapons you can use, and how and under what conditions you can use them. One of those restrictions, which we claim to observe but are not a signatory to, is the 1899 Hague Declaration, which forbids the use of bullets which expand on impact to create larger wounds. I'm not going to get into the philosophy or wisdom of the Hague Declaration except to note that we've always given it lip service, if little more, in the past. Even that may no longer be the case.
US Special Operations Command (SOCOM) recently purchased a new type of ammunition, referred to as "5.56 Optimized" or "brown tip" (because the tip is painted brown to distinguish it from other rounds), which appears to use a bullet based on the civilian Barnes Triple-Shock X hunting bullet shown in the picture below. As you can see, this bullet would be banned under the Hague Convention.
Bullets are designed for making holes in things, and from the military's standpoint the bigger the hole the better. The M16 and its variants fire very high velocity but small diameter bullets. Little holes aren't very good for killing people, but our standard M855 round, at typical combat ranges, fragments after impact to create a massive wound channel. It's perhaps the most effective and deadly small-arms round in the world. But the same round fired from guns with shorter barrels, such as the M4 carbine and some versions of the SCAR, doesn't have enough muzzle velocity to consistently fragment, making it much less lethal.
Enter the 5.56 Optimized. Expanding bullets are designed to work properly over a broad range of velocities, so they're deadly even when fired from more compact weapons better suited for urban warfare. The 5.56 Optimized is also "green" because it's made from copper rather than lead, which, believe it or not, is a major concern of our military right now.
And the Hague Declaration? Well, if necessary we'll just have to find a way around it. Considering that the M855 meets the letter of the Hague Declaration while violating everything it stands for, I can't imagine we'll let international concerns get in our way. Damn all those European humanitarians anyway. They have no idea what it's like to fight a real war!