Hi Blitz, Well I have never shot them at armor, but I have shot them at 2 inch thick metal plates etc. They did not penetrate, but did put a pretty good dent in it.

I believe the primary purpose of the SS109 round was additional weight. Back in the early .223 round stage, it was cosidered to light compared say to the brusing .308 WIN round. Despite the velocity, light cover would deflect the .223. Thus, in the 1970's they put a little steel in the core to help with cover penetration. As for a true armor piercing round, I think you have to up the ladder quite a bit....
Larry