It seems to be that part about being illegal to manufacture AP is behind the ATFs action against Barnes copper bullets.
Yes, that’s it in a nutshell. The ATF is entirely political and most capricious in its findings. They take law and stretch its interpretations to some of the most ridiculous left-sided rules one can imagine. They purport solid copper is armor piercing, yet still allow the manufacture and sale of XM855 (SS109) 5.56 mm Penetrator rounds on the commercial market (I bought two 420 rd cans of it from Walmart!).