People are, very, successfully shooting 6 BR's or one of its variants like the 6mm Dasher in 1000yd comps. so if you plan to build something that would be the direction I would go. If you're looking to buy an off the rack factory rifle and factory ammo has any kind of importance to you I still believe the 6mm Creedmoor is your best option because it is supported by easy to find very good factory ammo. Nothing wrong with the 243 but I think the 6CM has better factory ammo and arguably a better case design.