Re: Steel targets
AR400 and AR500 are "abrasion resistant" steel. @ 400yds They will both hold up to either of the two rounds you listed, they are very tough steel. AR500 is the tougher of the two and will last longer, but either will work. In all likelyhood, neither the 204 or 308 will even leave a dent @400yds.
Stuff like diamond plate and structural steel are generaly SA36, and fast bullets punch clean hole thru it like there ain't no tomorrow. I've hit my 1/2" SA36 targets as close as 450yds with my 270-300RUM with a 150gr bullet (mv @ 3500fps), and it leaves one hell of a crater but doesn't punch thru.