Don's being overly cautious, which isn't necessarily a bad thing with a relatively expensive gun. One of the instruction books I have includes a tag which states that steel shot should not be used IN FULL CHOKE BARRELS. Given the improvements in steel shot technology since the Repros were made, I would not hesitate to shoot steel through barrels choked no tighter than mod. (Especially on the 12's, I've found that the mod chokes are on the open side. They're WAY on the tight side on the 28's, but I don't think anyone is going to be shooting steel in a 28 anyhow.) I'd probably stick with nothing bigger than 4's as well.
Of course they did make a 12ga Steel Shot Special.
