Be careful shooting cast iron, if your too close you can get fragged by all the little chunks coming back. Even worse than shooting cratered steel.
That said a .556 will go through brake rotors a big truck flywheel will stop a 30-06 though.
I'm going to say this knowing that I'll probably get flamed, but here I go anyways.
About 35 yrs ago, I went shooting with my older brother and one his not too bright friend of a friend.
He shot at either a 5 or 10lb weight that I believe to be cast iron from about 15ft to 20ft with a 223 or maybe 556. I believe it was a Colt AR15
There were a couple fairly clean holes. I don't know if the bullet went thru or if the bullet knocked a plug out of it... if that makes sense.
That was the last time I went shooting with that guy because what happened was he tossed 2 of them about 15 20 ft and started shooting.
I was about 5 ft behind and off his left shoulder but looking at about 90 degrees to the left.
When I turned to see what he was shooting at.... a fragment of something hit me in the chin.
After some
we went and look at the weights. Saw 1 with a couple holes and the other had broken in into a cpl pieces.
That is the reason I only 1/2 heartily even consider 223 as a HD rifle. I know they're supposed to fragment in drywall but I also know what I saw 1st hand.
Not too oddly I guess, I wrote about this before, I think here or TFL.
The direct friend of my brother once had a 22lr go off in his back pocket when he fell on his butt roller skating after returning from a different shooting trip. Made a really red mark on his butt... bullet exited near the bottom corner of his back pocket and lodged about 1/8 below the surface of ply wood used to make a roller hockey rink.
Flame away