The chamber specs for 7.62 is a little looser than for .308, however its not going to be a headspace problem- the minimum headspace specs for 7.62 is a little more than .308, but well within the maximum SAAMI specs for .308.

The problems come when reloading. As mentioned before, some 7.62 brass is thicker than some commercial .308 brass, but this shouldn't make much difference if you size the cartridges correctly. M1/M1A/M14's beat the hell out of brass so you shouldn't use brass for more than 4 loadings anyhow. The casehead failures come when a reloader screws his sizing die all the way down to the shell holder, or worse, does the same using a small base die. The chambers on these rifles are big enough where a small base die should not be used. All that is needed is to set the shoulder of the case back far enough so that it will chamber with finger pressure with a stripped bolt. Any more sizing will weaken the brass and could lead to casehead failure because the shoulder is continually set back and the web of the casehead stretches and thins with each firing. The other extreme is not setting the shoulder back far enough causing the rifle to fire out of battery.


>308 ammo will work just fine in a 7.62 chamber and won't cause you any problems, provided that if you reload this brass you know exactly what you are doing.