Looked up .308 cartridge and chamber drawings on SAAMI (Sporting Arms and Ammunition Manufacturers Institute).
Found chamber drawing for 7.62x51mm NATO chamber, but not the cartridge. A comparison of chamber drawings suggests that datum line of the 7.62 NATO is .013-inch longer than the .308.
DATUM: HEADSPACE SPECIFICATION
DATUM----cartridge----measurement is taken from cartridge head to the middle of the shoulder of a bottleneck case.
DATUM----chamber----measurement is taken from breechface to the middle of the shoulder.
The .308 and 7.62 NATO cartridges interchange in civilian and military firearms. If the 7.62 case has a longer datum line, in swapping factory and arsenal rounds between rifles, I haven't found it. The NATO chamber is listed as having a .013"-longer datum line. Unless the NATO cartridge datum is longer, it means 7.62x51mm NATO is meant to stretch. Realize, that military ammo is not designed for reloading, neither is all civilian brass designed for reloading.
Military brass has thicker walls for use in loose chambers, particularly machine gun chambers. As a contributor stated, a head separation hazard exists when commecial .308 is fired in a 7.62 NATO chamber, which allows for .013"----longer between breechface and chamber shoulder.
SAAMI
.308 Win----62,000 PSI piezo. Proof----83,000-89,000 PSI piezo.
NATO
7.62x51mm----58,000 PSI piezo. Proof----67,000 PSI piezo.
RELOADING
I have reloaded GI as well as commercial brass for decades without a problem. I prefer GI 7.62 brass in applications where I don't want to fill the case, notably various silhouette cartridges, such as the 7mm/308x1-3/4".
When I loaded for the .44 Auto Mag, I made the rimless case from GI 7.62 and .30-06 brass, a rather tedious process which, besides trimming, required inside reaming.
Springfield Armory, Inc., M1A
I have shot these very accurate rifles with with commercial as well as military ammo, without a hitch. Most handloads have been with U.S. military brass. As with a few other military rifles, to prevent potential slamfires, use of military-type primers (heaviet cup) is stronly recommended. If I get any dope as to which spec the M1A chamber is reamed, I'll pass it along.
While the M1 Garand is an obsolete, and the M1A nearly so, no battle rifles so combine their POWER, TRIGGER, SIGHTS, and ACCURACY, with RELIABILITY. That combination is never obsolete.
David Bradshaw