As far as I can tell, the "SS" is just FN's designation for their bullets, followed by the specific numeric identifier that determines bullet construction/caliber. SS-109 designates Fabrique Nationale's 61.5 grain bullet with a steel penetrator in the nose. The SS109 is not necessarily the same as the US military's M885 round - our military has additional specs that the complete ammunition must meet before being designated M885.
DocChronos's link provides weight and bullet construction details for the powder-puff 5.7x28mm stuff that FN makes, although the chart seems to have omitted SS192 :?. Don't get any ideas, babj :evil:
With the steel penetrator in the nose, it would almost certainly pop a second chance vest, though a ceramic chest plate would probably stop it without much trouble. There are different classes of body armor, as you are probably aware - .223 isn't going to plow through them all.
And it is probably worth mentioning that we should be very careful with where this conversation is going...
Short answer, it probably is. Not all "SS1**" is "armor piercing" - most of the 5.7 stuff widely available is not. See this thread for more info on the different flavors of the 5.7x28mm.
My advice - stick to 5.56x45mm, 7.62x39mm, or 7.62x51mm if you are looking for for a rifle that fills the niche.