My experience is similar to kuduae's--the groove in my rifle measures .268, the land .255, but both the .264 Hornady RN and factory 159 gr. RWS T-Mantels shoot fine. I also get decent results with the Hornady 140 SP, although they don't feed as well. I'd like to try the 155 gr. Lapua but haven't gotten around to it.
I know some people use the Hornady Carcano RN (.268 diameter) if they can't get the .264 to work, but I suspect that mainly happens in guns with pretty weak bores, which yours almost certainly doesn't have. I'd definitely try the standard Hornady RN right off the bat, see what happens, and go from there.

Good luck, and keep the info coming!