I would agree with Mr. Sorensen that the Remington 700 would be my first choice for an accuracy platform. Unless money is no problem then I would opt for a custom action. ( no action truing) I think it is unfair to say that a Remington 700 requires a great deal of work to achieve their potential. It has been my experience that they require less than most factory actions. And they are alot easier to work with. I think thats why they are so widely accepted. The straight forward action was designed by an early accuracy shooter and friend Mr Walker. I would go as far to say if you took a poll on BRC that the 700 would be the first choice of most of the shooters if they had to start with a factory action.
Another point would be that Custom 700's sell quicker and for more money than just about any other factory action.
As Mr. Sorensen touched on, There are alot of after market performance parts for the 700. Just look on Dave Kiffs website under Remington Performance Parts. Try finding them for any other actions.
As far as chambers for 600 yds, Do you plan on shooting paper, Varmints,or long range hunting?
Boe