AR500 is quite a bit harder than mild steel. As an example, Salute Targets test their AR500 with 30-06 and 50BMG Armor piercing rounds. I figured the steel core might put some little dimples in the steel, at most, but not put holes in it.
The whole point of the steel penetrator is to penetrate
That's what it was designed to do.
The only real difference between "armor piercing" and "steel core" bullets, is that "AP" bullets have a hardened penetrator. Regular "steel core" bullets have an unhardened penetrator.
It matters to the ATF. But, in reality... they're both steel cylinders; and react in much the same manner.