IIRC, the ATF definitions of illegal AP rounds are bullets that may be used in a handgun (AR pistol), and have:
-More than 50% bullet weight in the jacket
-Bimetal bullets using brass, tungsten, depleted uranium, steel, ect. as core material.
-Aaand some other stuff I'm not sure of. but the above is pretty much the catch-all for AP ammunition.
Though, I've seen .223 solid brass slugs for loading 5.7x28. I think there is a loophole that if the slug is a solid material it's okay. Of course, the harder the metal, the more wear on the barrel...
In general, .223 is not a good anti-material cartridge. The steel penetrator in M855 and 6n7 ammunition isn't considered AP by ATF, but they aren't really effective against vehicle armor. You'd need to go to some spicy .308/7.62-Nato rounds. But then again, thanks to some select idiot yahoos, there are .308 pistols, so no AP for .308 either...