In my very limited experience 400 is spot on. With a scope you generally have to choose whether the rifle is regulated with the scope or without. G. Wright in his book mentions that with one rifle he shot if he put the scope on the groups "stacked" one over the other. That's been my experience. So a DR that's not been regulated with a scope but has a scope put on will often open up the groups. Now, if the rifles been regulated with the scope the groups will probably be smaller than had it been with iron sights. Also with a scoped small or medium bore you could have it regulated at 10 yards rather than 50 or 75. However, there are not many "rules set in stone" with DRs though...

Note: Thanks Bulldog, yep that 100 there. Well I suppose you could do it at 10...