(X)M855 is a horrible choice for SELF DEFENSE. Biggest reason being the imminent pass through risk.
If you're planning on going to war, THEN, it would be a great choice.
Heavy projectiles such as the 77gr SMK(MK262 MOD O) are great for extended distances. Far beyond anything that would be constitute reasonable "self defense" distance.
A 40-50 grain HP would be ideal for upclose work. Something that's going to come apart on impact, explode, if you will... minimal chance for shoot through.