There are no restrictions on the ownership of rifle caliber armor piercing ammo that i'm aware of. Folks own thousands of rounds of armor piercing rifle ammo that never hurts anyone.
In the mid 90s the US Army developed an armor piercing round in 5.56mm-the M 995. Not sure that round was ever fielded: i've never seen one round of that ammo at gun shows or anywhere else. Ammo peddlers often call the SS109/M855 ammo "armor piercing" but it ain't.
http://www.fas.org/man/dod-101/sys/land/m993.htm
http://www.gunbroker.com/Auction/Sea...=3017&Items=50