I know for a fact that a 527 can be converted to 6.8. A few years back when it first came out I read an article in Buckmasters Rifle Hunter magazine featuring a custom 527 in 6.8spc. But maybe they used a modified .223 bolt, I have no way of knowing.
Another possibility is a 6mmAR, basically just a 6.5 Grendel necked down to 6mm.