Interesting - but I thought you could get armor piercing rounds for rifles? (Or have I just watched too much Miami vice?)
Almost any rifle round is “armor piercing”, so rifle rounds (at least the big ones) are not subject to the law.
The Obama ATF made the Russian 5.45×39mm cartridge (a rifle round for the AK-74 (yes, 74) illegal because it is designed to defeat a NATO Kevlar vest — despite being a rifle round because there was some prototype pistol that could shoot it.
Tried to do the same thing to the 5.56 because of AR-style pistols, but that failed.
Not sure in the 5.45 ban still stands.