RE: 30-06
Semper Fi
Shot placement is the key. If you and your rifle can produce consistant 6" groups or better at 400 yards, the wind conditions are managable, with the correct bullet you should not have a problem at 400 yards with a clear line of sight. You fire on the range at 500 yards with open sights from a .223, no reason why you can't do the same with a different rifle. Still you should do everything you can to ensure a clean kill, including getting closer. Most hunting situations will not call for such a distance.