Quoted:
It's MILD steel, and simply makes the bullet cheaper to manufacture. It might penetrate SLIGHTLY more than regular lead ball, but it's not AP.
Can you provide information on the relative cost of production of lead vs. steel in any country that produced steel core bullets, along with the source of that information? Certainly, producing a three component bullet wasn't cheaper than producing a two part bullet, unless the steel was much less costly to make than the lead.
ETA: It would be wonderful if you would tell us all about your experience testing the relative penetration of steel core and lead core bullets. That way we'd know what your definition of "SLIGHTLY" is.
I agree that steel core isn't AP and wasn't intended to be AP (even though I've never tried to penetrate armor plate using it).