The SS109 is not legally AP while in the 5.56x45mm, because the 5.56mm is not a pistol round.
The SS109 DOES, however, have a steel core. Thus, if you load it in 7.62x39mm, it is AP.
I know this is stupid, but the ATF says so. If the core isn't lead or a "legal lead substitute" then its AP... or if the bullet jacket makes up more than 3/4 the bullet... something like that. It is really stupid, but I can look it up for you if you want to read it from Big Brother himself.
EDIT: OK, any jacket that makes up more than 25% of the bullet, not 3/4. Also, SS109 was taken off the AP list, but it still has a steel core, which means loading it in 7.62x39mm would make it illegal. The .30-06 black tip AP ammo has a steel core, is not considered AP, but would still be illegal for you to stick that steel core bullet in the 7.62x39mm.
http://www.atf.gov/firearms/legal/armor.htm