M855 on AR 500 at 200 and 300 will usually just "peen" the surface but its not the target a lot of clubs are concerned with, its that wicked little hardened penetrator. The regular fmj, sp and hp ammunition just explodes into powder and fragments. The steel doesn't.
While the peening of the surface is not really that bad most clubs don't like to risk the damage to expensive targets.
Just a note 5.56 generally does more damage to steel than .308, 30-06 ect. due to the high velocity and small point of impact.