The steel must be flat and smooth. Any imperfections in the surface can cause bullet frags to be re-directed back to the shooter.
Mild steel is OK for velocities under 1200 fps AND OVER 750 fps. but mild steel will always be hit by some slob with an AK even after you tell them not to. The resulting damage creates a dangerous situation that can / will cause injury. That is why we always recommend AR500 for shooting targets.
