Mine is 6.5 Grendel.

I have two that have shot every type of factory ammo I've tried accurately. That's my .308 and my 6.5 Creedmoor. I would have to give the nod to my .308 overall because I've shot several more types of ammo through it. I've only shot two different factory rounds through my Creedmoor so I can't really judge them equally.

.308

Just my .02,
LeonCarr

I suspect you're assuming that most of those factory loads for the .308 have different bullet weights. It has been my experience that you most often see a small, subset of bullet weights that every ammo maker seems to offer. I suspect it's because after decades of hunters and reloaders finding what bullet weights shoot best, ammo makers focus their production efforts on those findings. For example, after more than 100 years since its release, 150 and 170-grain bullets seem to dominate the 30-30 ammo market. Just about every ammo maker under the sun who makes .270 ammo offers a 130-grain load. As for the 6.5 CM, it seems if you're not shooting Hornady's 143-grain ELD-X loads, you're obviously not in the know. With the load being a relative newcomer (2007) in the shooting industry, that may change over time.

the rifle is a much more significant variable than the cartridge.

Never had a problem getting any of mine to shoot well from 223-300 wby and 45-70. They are all fool proof.... most of the times the fool is the problem not the rifle or bullet or etc.