From the article Blue Wolf is referring to:
There appears to be some confusion as to if and when it is appropriate to turn steel targets over, in an attempt to get additional life out of them. It is our intent to clear that issue up, and share some general information with those tasked with range equipment maintenance.
To apply some context, years ago the steel being used for targets was primarily T1, manufactured by U.S. Steel. It had a Brinell hardness rating of about 235. When it was shot a lot, it would become concaved (i.e. dented in), in the impact area.
The steel we use at MGM Targets today has a Brinell rating of 500 and does not become concave when shot. 500 Brinell steel is much harder and consequently, the result of bullet impacts is dramatically different.
Envision a penny on an anvil being struck repeatedly with a hammer. It gets much thinner, and at the same time, larger in diameter. Exactly the same thing is happening to the face of our pistol targets as we pound them with bullets. The hardness of the current steel allows only the front of the target to be peened, (enlarged). Therefore, if the near side of the target is growing (from the constant hammering) but the back side does not grow, the only thing that can happen is for the target to bow. It will often become CONVEXED, and bow toward the shooter - Yes, exactly opposite of what T-1 targets do.