XM855 isn’t for penetrating soft armor. As already stated, any standard 5.56 round will penetrate IIIa soft armor or lighter. The primary reason the military designed the XM855 is to penetrate an old steel military helmets at 500 meters, since the ball they were using at the time couldn’t do that.
My whole purpose of pointing out the XM855 was because that will be the first round the politicians will outlaw because of its ballistic characteristics. All you need is someone in the legislature to say that all steel core ammo penetrates all types of armor, and the Libs will institute a moratorium on all sales of it in addition to banning it.