Old thread but I have some answers if it means anything at this time.
The French MLe M.A.S 49/56 is a very well built semi auto rifle.When Century imported these into the US they converted some over to the 7.62X51 .The problem with some of their conversions was a rough boring of the chamber.When .308 rounds are fired they expanded in the chamber and caught these rough ridges that were left from the poor machining process..Some say Century used really tired reamers to complete this process thus causing the issue.
I have a converted model and have never had an issue firing .308 through my rifle.The one thing I do know is you want to be sure to be on a range when loading the chamber as these rifles have been known to slam fire.Military primers are harder and the firing pin under heavy inertia would not activate the primers.
Also this rifle was designed for use of steel cases not brass so the extractor is very heavy and has been known to rip the lip off of the shells when firing brass cased rounds.
I mainly use mil spec ammo but have not had any problems shooting civilian based ammo..These rifles from Century truly can be a hit or miss as with most rifles from Century arms.Its always best to be able to fire one and inspect it before purchasing if possible..You may get one like mine that performs flawlessly and end up with a real keeper..Or end up with a rifle that will irritate you to the point of pulling hair out..
Good luck if you ever decide to get one..If you get one that works I promise you will be extremely pleased with it..