More specifically, the SS-109 bullet has a mild steel penetrator sitting on top of a lead core.  This "compound core" is sheathed in a copper jacket.  Nominal weight is 61.5 grains, but due to the compound core, the bullet is as long as a 71gr lead core would be, and requires at least a 1:10 twist to stabilize.

This round was developed by FN of Belgium for use in the FN Minimi belt-fed 5.56 machine gun that was eventually adopted by the US as the M249 Squad Automatic Weapon (SAW).  As part of the SAW requirement, the US military wanted ammo that penetrated better at long range than M193 (55gr FMJ leadcore) ammo did.  The M855 round can penetrate both sides of the then-current *steel* helmet at 600m.

More recently, an AP round was developed, called M995.  This ammo has a black tip, and the core is tungsten, which is denser than lead but much harder.

-Troy