600 isn't that far. A 7mm Rem Mag, 7mm STW, or 7mm RUM would easily take care of it. Just make sure to load them up with some good high BC bullets, either Bergers, Wildcats, or Accubonds. If you want something that will pretty much outshoot ANY factory chambering look into the 7mm Allen Magnum. I see nothing a .30 cal will do over a good 7mm, if you need something that big look into the .338 RUM or .338 Edge.