Actually, M855 was designed to penetrate the Warsaw Pact standard helmet from the time of its adoption better. Nothing more, nothing less. It's not any more 'armor piercing' than M193: either round will penetrate soft body armor designed for pistols and be easily stopped by hard armor rated for rifles. The penetrator in M855 tends to leave a tiny dent in hard armor, nothing close to penetration.
Folks calling it 'armor piercing' is what almost got it banned under the Obama administration ten years ago.