7.62x51...
If you're thinking of 7.62X51, in a semi-auto platform, please think of several things:
Ammo- is it full powered, or is it limited, as is found in M1 Garand and M1A rifles? Yes, M1 Garand... Both .30-06 and .308NATO(7.62X51) in the M1 Garand are "underpowered," as in not hunting rounds found in bolt action rifles. Too high a power, destroys weapon, and endangers you.
Same for M1A. Add to that, M1A can be torqued by not holding properly.
As both rifles are gas operated, they are more delicate than bolt action rifles, or rifles of the CETME, HK, DSA, or PTR type roller delayed action rifles.
AR type rifles have only a few that can operate with full caliber ammo. The new LMT 7.62X51 adopted by the Brits is one. Knight Armaments has another. Both are on the AR platform, and both use pistons. HOWEVER, they are not as robust as the PTR, which is a modern build, like they are...
IF you must have an AR platform, LMT and Knight are the best- PERIOD.
Me, I'm a diehard PTR MSG-91 fan in 7.62X51, and have a PTR 32KFR in 7.62X39. I've owned FIVE M1A Super Matches, and a match grade M1 Garand. I won't go back...
Hope this helps...
Glockcurmudgeon, out...