My first CETME was made from a well worn kit, and it would shoot commercial .308 all day, even though the chamber was fluted. It bought a second one made from a new kit, and the commercial .308 hung up in the chamber. It shot thick walled NATO brass rounds just fine.
Head space on the 308 is a touch longer.
The older gun might have had enough slop worked in the allow the 308 to “fit”. The new gun was too tight for the slightly longer 308.