The U.S. Army has been using the new M855A1 5.56mm round in Afghanistan for nearly a year now, and apparently the new ammo is an improvement on the older M855. A year ago, the army and marines parted company over what kind of rifle ammo was best for the troops. The army had just begun buying 200 million rounds of the lead-free M855A1 5.56mm ammo and shipping it to the troops. The marines went with a slightly more expensive Mk308 (or SOST) round, that SOCOM (Special Operations Command) recently developed. SOST contains lead, but SOCOM considers it more effective in combat than any other 5.56mm round available. For the marines, this was pitched as a temporary measure, until the M855A1 was available for them, and there was some combat experience for the M855A1 to prove what it could do.
One reason for the appearance of the M855A1 was years of political pressure on the army to use non-lead bullets. That came about because training and combat use of army 5.56mm weapons puts 2,000 tons of lead back into the environment each year. This lead was originally taken out of the environment to be temporarily stored in the form of bullets. But the M855A1 was also about several more fundamental needs.
The M855A1 contained a number of other improvements that the troops wanted. For example, the M855A1 is a little more accurate at longer ranges. This is important in a place like Afghanistan. The M855A1 is better at blasting its way through brick, concrete and masonry than the older M855. The propellant in the M855A1 burns faster, and thus produces a smaller muzzle flash when fired from the short (compared to the M-16) barreled M-4 rifle. The greater penetrating power of the M855A1 is because of a steel penetrator, which also makes the M855A1 more likely to penetrate body armor and sheet metal. The Taliban are increasingly getting their hands on protective vests, or adding armor to vehicles (particularly suicide car bombs meant to speed past armed guards).
While this non-lead policy burnishes the army's image and environmental cred, it was also feared that it might equip troops with an inferior bullet, which was built around a copper alloy (not lead) slug. But inferior to what? Well to a another new bullet. SOCOM (Special Operations Command) has developed a new 5.56mm bullet, the Mk 308 SOST (Special Operations Science and Technology) round. The SOST bullet solves a problem the M855 has long had, the inability to penetrate things like automobile windshields. SOST uses lead, and also has more killing power than the M855 (that did not inflict as much internal damage, and bleeding, as 7.62mm and 9mm rounds.) The M855A1 turned out to perform these tasks as well, or nearly as well, as SOST, and was still "green" (less polluting).
The marines always planned switching to the M855A1, but used the delay in getting the new ammo to try out the SOST round in combat. Soldiers wanted to try the SOST round as well, but the only army personnel getting that are Special Forces troops. It turned out there was not a lot of difference, in performance, between the two rounds. SOST costs more, and is marginally better in some situations. For SOCOM, that's reason enough to go with SOST.
The army has spent over $32 million developing the M855A1. SOCOM spent a lot less developing SOST, which has a bullet that weighs as much as the M855A1 slug, but is based on a popular hunting bullet design (the Federal Trophy Bonded Bear Claw). SOCOM is constantly searching for better weapons and ammo. The army, however, has ten times as many troops and buys a lot more ammo. The army also has a larger procurement bureaucracy and heightened sensitivity to media and political criticism. SOCOM can get away with saying "it's none of your damn business" and continuing to experiment. This is especially true after SOCOM troops went and killed Osama bin Laden.