Hard to tell, honestly. Most manufacturers stick to a known set of dimensions (not like the TDP for an AR-15, though). The .308 ARs have a pretty common set of dimensions, despite the three different basic design styles (DPMS, Armalite, Rock River). The most common problems come up with the "custom parts." Biggest issue to date was with Iron Ridge uppers and lowers, even though there are small things one in awhile. Even though Wash just had an issue with a 6.8 barrel and a Mega monolithic set recently (which is a 5.56 AR-15 setup), it's usually not a problem, and a problem exists when the barrel won't fit in the upper. Tight fit is fine - that's routine, and actually desired. Too tight to fit is a problem, tolerance-stacking aside.