Here is what happens when you shoot 5/16″ mild steel plate with various rifle rounds from 25 yards.
On the left is a 69 grain BTHP .223 with a velocity of roughly 2600fps; it fails to penetrate the plate. Next is the impact from a 55 grain FMJ .223 (about 3100fps) which punches clean through. I even found the divot it knocked out, which I placed below the hole for the picture. On the far right is the hole left by a 150gr FMJ .308 bullet.
Here is a view from the back of the plate.
On this angled view you can see the size of the divot, and also the dent made by the round that failed to penetraate.
Here is the side view.