couple of questions:
What is your intended use? Gongs, paper, varmints can all influence round choice.
Do you reload? If not, long distance shooting is going to become very expensive very rapidly
What do you consider long range?
Are you prepared to spend, at minimum, the same amount of money on optics as you are on the rifle?
Do you have facilities to practice long range shooting?
None of these questions is meant to dissuade you. Just a few realities I have encountered when gearing up for my own long rane experiments
I highly suggest heading over to 6mmbr.com and researching the various long range rounds and their ballistics.
With the correct barrel and loads, even a .243 will get you out to 1k yards, without beating your shoulder into a pulp.
You might also want to look into the 6.5x284. You can purchase an F-class rifle and ammo off the shelf and be in the distance game in a matter of minutes.
Good luck...but watch out. Next thing you know you'll be fire forming brass for a 367 limpwrister shoulderbuster wildcat! The distance game is addicting.