From Armalite: All of our AR-10 .308's are NATO chambered and will accept both 7.62 and .308 ammo. The AR-10 can shoot 308 OR 7.62 NATO.
The difference between .308 and 7.62 is chamber size, slight pressure differences due to powder burn rates, not external case size. Both cases will have the same external dimensions while the .308 has a slightly higher pressure 60Kish vice 50-55 CUP psi for NATO**.
**As a side note the reason the pressure issue is confusing is becuase the military measured 7.62 NATO chamber pressure (PSI) via CUP (in the 40s) vice the normal commercial rating of PSI measured (now) with electronic means. You will see many cling to the 50-55K PSI figure for 7.62 Nato becuase that is what the old Army TMs have in them. Since there is no conversion for the CUP measurement to the newer electronic measured PSI, they are two different numbers i.e no direct comparison. Many people will assume the CUP PSI is the same as SAAMI/Commercial PSI. It is not. Commercial PSI is derived from the direct pressure in the chamber through electronic means (piezo transducer or strain gauges). CUP is a crush measurement of a copper slug that has been fired, again not the same test.