Chris NZ is spot on here also, good points Chris.I'd think either stick with a 6BR for efficiency or else go to a 6mmRem is you really feel you need a bigger case. The 6mm Rem is a nicer case design than the .243, ie a longer neck which will give better alignment, more flexible seating for heavier projectiles in faster twist barrels and is nicer on barrel throats.
Chris NZ
Dave T