The M855/SS109 green tip ball ammo is not a true hard armor penetrator at all. It's purpose was to provide better penetration through light cover and non-hardened structures like truck/car doors and typical light building materials. They also are designed to improve penetration through any soft body armor and gear worn by enemy troops. It will not penetrate any practical thickness of genuine hardened armor plate. On impact with any truly hard material the jacket would separate, and the mass of the steel insert is very small and is not itself hardened steel, which would provide little additional penetration.

Mike