If you go to the ATF website and punch in keyword "armor piercing" you will pull up a page that lists what is and what is not considered "armor piercing"
Specifically, the ATF listing of "armor piercing" ammo seems to concern only a certain few handgun
rounds namely, some .25 ACP and some 9mm (9x19) pistol loads as armor piercing due to the fact that they are constructed of certain materials that when fired from acutal pistols can defeat some but not
all soft armor vests.
Included were: .25 ACP by MSC industries; another from some firm in Georgia and a few obscure 9mm
para. rounds I've never heard of. I don't recall offhand if the much maligned, news media monster
KTW was on this list or not. The point is moot since KTW was NEVER sold via commercial channels
anyway since 1972. Furthermore, to the best of my knowledge no police officer has ever been shot with one. (Thanks to an early 1980s report by NBC's Jack Perkins, the media has successfully educated criminals in what works and what doesn't and this is reflected in FBI UCR crime stats on officers shot
since then)
What is specifically EXEMPT from the definition of "AP" are 5.56mm "SS-109" ; "green tip" or equivalents and caliber .30, "black tip" "AP" ammo (which was at one time, the preferred long range .30 cal match ammo due to the heavier 165 grain bullet's performance and accuracy at longer range
stages of national match shooting before the advent of modern, match ammo)
It is interesting to point out, however, that 7.62 NATO M-61 "black tip" which is essentially the same bullet, is NOT EXEMPT and IS considered to be illegal, "AP" ammo.
Why that is I can't say for sure. It probably has to do with the CMP or DCM years ago. Since match shooters shot .30-06 black tip all the time, it was circulated in distribution whereas I don't believe
the .308 black tip AP ever was. That's the only rationale I can think of
since their practical performance is identical *about 1/4" steel plate penetration at 300m and 1/5" at 500m. They are identical in construction, and almost identical in weight 148 gr vs 150 gr and velocity 2780 fps nominal for 7.62 and 2850 fps nominal for .30-06.
Really stupid laws!