I posted this in my barrel build thread, but will share it here too just for fun - this is a sample of 5/16" Peterbilt frame rail (this stuff is high strength but not hardened). 32gr 20 Tac (mild load, about 3900 fps) on top vs 75gr 223 Wylde (~2600 fps) on the bottom.
High velocity makes a big difference on steel.