The Grendel is quite accurate, and has been used to take quite a few deer already, along with hogs, etc.
Heres one of the latest:
The 6.5 Grendel Forum :: For the 6.5 Grendel Aficionado - Grendel bags big buck
and an antelope thread:
The 6.5 Grendel Forum :: For the 6.5 Grendel Aficionado - Grendel fills antelope tags
I use mine for varmints, but loading the 120 grain Barnes TSX it turns into a pretty good deer round out to 300-400 yards, assuming you can put the shot on target.
As far as the 6mm BR goes, well, thats an interesting choice for hunting, most of the rifles for it are pretty heavy to carry around for any distance!
6.5-.284 or the 6.5x47 are also good rounds with more pop than the Grendel or BR.