I'm going to give you the very boring and disappointing answer. In the early 80s, the military decided that their 55 grain M193 was not providing sufficient penetration. Primarily against things like helmets and car doors and for use in the SAW.

So it was designed to pierce soviet helmets at range, which at the time were a thin steel design. And that's what it does really well. It goes through minor metal barriers quite well.

It doesn't pierce plates/body armor any better than M193, but it does mean it will retain more energy when passing through barriers. Oddly, this makes it not a particularly good choice for any practical applications as a civilian because it is much more prone to over penetration.