[andyk]

Technically speaking, any type of ammunition is legal in the United States so long as it's not armor piercing, according to Earl Griffith, firearms technology expert for the U.S. Bureau of Alcohol, Tobacco and Firearms

I call BS. Even AP, APT, and APIT .50 BMG rounds are legal for purchase.

[USNBandit]

I think they are talking about not the ability to punch through armor, but rounds made specifically to go through armor. A rifle round will definitely punch through a soft vest, legal. Armor piercing FN 5.7 rounds made for punching through armor, illegal.