A new round replaces the current M855 5.56mm cartridge that has been used by U.S. troops since the early 1980s. The M855A1 offers a number of significant enhancements: improved hard target capability, increased dependability, consistent performance at all distances, improved accuracy, reduced muzzle flash and a higher velocity. It’s tailored for use in the M-4 but also improves the performance of the M-16 and M-249 families of weapons. (U.S. Army)
http://www.tactical-life.com/online/news/m855a1-green-ammo-shipped-to-afghanistan/
http://www.tactical-life.com/online/news/m855a1-green-ammo-shipped-to-afghanistan/