From Record Online:

The challenge is that there are handguns designed to fire certain rifle calibers and that rifle ammunition wasn’t designed for anything other than sporting purposes. That means you could have a rifle caliber ammunition type outlawed on the basis that it can be fired in a handgun and pierce armor. That covers a wide range of rifle ammunition that could be outlawed under a stricter interpretation of this law.

According to the NRA representatives who attended the meetings, the ATF “suggested that it believes that the armor piercing ammunition law was intended to affect all ammunition capable of penetrating soft body armor worn by law enforcement officers.”

More at the link.