Black tip (M995, I think?) is armor piercing, and it has a tungsten rather than steel core. M855 is just regular old ball. Ball will still punch through mild steel pretty well, however.
Furthermore, armor piercing rifle ammo is perfectly legal in most cases; look at all the .30-06 AP in civilian hands. So-called "armor piercing" pistol ammo is illegal, thanks to the gun-grabbers' panic mongering about "cop killer bullets"; then again it doesn't really take too much to ventilate soft body armor. When I think armor piercing I think anti-material purposes - even a .30-30 soft point will penetrate the soft armor police wear.