SHotgun hits do really wierd things to steel.
all of the steel shotgun targets the match uses are armor plate, and they don't last that long. the plates start taking on a curvature almost immiediately (we're still not sure why this happens) the plate edges either curve away from the shooter, which is just fine, or they curve towards the shooter, forming a parabolic lead projector.
I think any lead shot of any size would be just fine on steel.
I've heard of some nasty injuries from people who inadvertantly chamber a slug and hit a close (50 yards and under) steel plate with it, since the slug likes to spew about an 80 grain ghunk of lead back along it's course.
it's not always easy to tell slug from shot anymore, since some reloaders use a normal star crimp with their slugs.