Great! That should introduce some new chalenges.
At 400 yds. you're really pushing it with 223, depending on the target size, angle of engagement, etc...
At that point, you might be talking about a 20" bbl model, but that poses problems for the "action" part of three gun action matches. Going through mock-up "doors", etc...becomes more of a challenge withthe extra 4" of barrel. I would still stay with an A4 and a free-float tube or quad rail, and consider our new mount when it becomes available.