Another point to consider with the .308 ARs is parts don't mix-and-match well between manufacturers like they do on the AR-15s. They're not built to the same dimensional specs. At least that's what I gather from manufacturers wesbsites stating so.
From RRA's website:
http://www.rockriverarms.com/index.cfm?fuseaction=category.display&category_id=451
From RRA's website:
http://www.rockriverarms.com/index.cfm?fuseaction=category.display&category_id=451