From Bearing Arms:

SS109/M855 is one of the two most common loadings for 5.56 NATO chambered AR-15 rifles, featuring a 62-grain bullet with a mild steel penetrator core. The cartridge was adopted by NATO in the late 1970s to give soldiers better long range performance, and reduce the possibility of fragmentation seen in the prior 55-grain M193 round cartridge that some viewed as “inhuman” and “devastating.” Civilian shooters like it because it is accurate, plentiful, and relatively economical to shoot.

ATF letter here