Try another approach, what about optics that compensate? Personally I would stick with the 30/06, 180 grain Barnes TSXand buy the Leupold VX-III with the Boone & Crocket Big Game Reticle.

Or if burning powder is your gig then get yourself this big bore: the .416 Meteror!! http://www.lazzeroni.com/ct_lacart.htm

Actually, I just read the ballistics and it drops too much at 400 yards.. Never mind..