The U.S. Army and Marine Corps stopped using the same 5.56mm ammunition in 2010 when the army began using the new lead-free M855A1 5.56mm round for combat (mainly in Afghanistan) operations. The M855A1 replaces the older M855 long used by the marines as well. The marines also wanted to adopt some new and improved 5.56mm ammo but decided the M855A1 was not it and continued looking. Now the marines believe they have found their new round, the M318 SOST, which has been used by SOCOM for several years. This annoyed a number of key people in Congress who insisted both services use the same 5.56mm ammo. To decide the issue there will be tests, and possibly more tests after that. In the meantime the political winds may shift and make the Congressional mandate for common ammo go away. After all for decades SOCOM has been allowed to use whatever ammunition (or weapons) it feels are best for the job at hand. But Congress considers SOCOM a special case, at least more special than the marines, and leaves SOCOM alone.
The new M855A1 round is more expensive and marginally better than the older M855. The major 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. The lead is also contained in a copper jacket and most of it stays that way. That's probably why no environmental study has ever found lead leaching out of spent bullets and getting into anyone's water. But just the thought of all those billions of lead bullets lying in the ground mobilized an international movement to ban lead bullets.
Fortunately the M855A1 was also about several other improvements besides being lead-free. For example, the M855A1 is a little more accurate at longer ranges. This is important in a place like Afghanistan. The M855A1 is marginally 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 were increasingly getting their hands on protective vests or adding armor to vehicles (particularly suicide car bombs meant to speed past armed guards).
The marines noted that the steel M855A1 bullet also caused more wear and tear on rifles using it. The marines noted that the M855A1 has been causing cracks in rifles that have fired as few as 3,000 of them. Marines also point out that the M855A1 requires larger safety zones for rifle ranges because the M855A1 tends to ricochet farther. The larger safety zones could be very expensive for some marine ranges.
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 another new bullet, the ones the marines want and SOCOM (Special Operations Command) already uses. SOCOM developed the new 5.56mm M318 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 potential lead pollution).
The army spent over $32 million developing the M855A1. The new bullet is more expensive (because of the more complex manufacturing process) and in the field troops have not noticed much difference. On the down side, the new round generates more pressure in the chamber (and higher speed leaving the barrel). In theory this causes a slight increase in the risk of a rifle exploding. That has not happened yet.
Some green bullets have been disasters. Norway introduced one three years ago that made users sick. It seems the new bullet, when used in new rifles, created some toxic gasses. A redesign of the new round fixed the problem and made the new bullet even more expensive. The marines believe the M318 is superior to the M855 and the lead-free M855A1 and has already shown that to be the case in combat (mainly with SOCOM). But Congress cannot be ignored and the marines will have to wait.