They aren't usually steel core unless they are old *** surplus, the cheap ***is steel jacketed. If it says "bi-metal" it's a lead core with a steel jacket that is plated in either copper or zinc.
It apparently damages backstops but I find that unlikely. More likely that the ban came about when surplus AP rounds we're available.
Ill check it out when I get back home. Heck maybe even PM you a picture of the box. I just know I can't use it at some ranges, my preferred one at least. It was really one of my stupidest moments of "not reading the box" moments. =/
But doesn't mean I can't use them in a pinch, so that's why they are in my mags now. lol.