Re: F-Class vs BR out to 600yds?
For all intents and purposes... the two guns (mid-range F-Open rig, vs. 600yd BR Light Gun), regardless of caliber, should be more or less interchangeable. If both are truly 'top shelf', the parts list and assembly will be basically identical. About the only difference would be that a BR light gun is limited to under 17 lbs, vs up to 22 lbs for F-Open...
As far as the cartridge choices... the 6 Dasher is pretty well established, vs. a relative new-comer in the 6 Creedmoor. Not saying the 6 CM won't work... but if'n I were spending my money and planning on hanging with the big dogs in a mid-range F-Open match... I'd hedge my bets towards the known quantity of a Dasher.