http://yarchive.net/gun/politics/armorpiercing.html
3) USE - The bullet must be able to be used in a handgun.
Rather than
construing this to mean regular handgun calibers, ATF construes this to
mean any caliber for which a handgun has been made, including handguns
in rifle calibers, like .308 Winchester, and 7.62x39, for purposes of
bullets covered by (B)(i). Thus bullets suitable for these calibers,
as well as other rifle calibers for which handguns have been made (at
least commercially made) which are constructed as described below would
be AP ammo.
However bullets that fall into the AP definition under (B)(ii), because
their jackets comprise more than 25% of their weight (solid copper bullets?)
must be intended for use in a handgun, not just be able to be used in a
handgun.
4) CONSTRUCTION - The bullet must either have a core made ENTIRELY out
of one or more of the listed metals, or be full metal jacket type
bullets with a jacket comprising more that 25% of its
weight. Thus SS109/M855 .223 bullets are not covered,
because their core is only partly steel, and partly lead. Lead
is not a listed metal, and bullets with cores made partly out of lead
are OK. ATF has expressly ruled that SS109/M855 bullets are not
covered.