Quoted: The M1A vents excess gas, while ALL the gas is used in operating the M1.
Given milspec .30-06 and 7.62 NATO ball, which fire roughly the same weight projectile at the same MV, it's a toss-up. The cartridges' energy is almost exactly the same.
The M1 has a more violent action, but then the M1A is (usually) a bit lighter.
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You've touched on what I was going to add here,
raf.
The projectile weight is the same. The muzzle velocity is the same, ~2700fps. But the .30-06 has a larger case/chamber, and can be safely loaded to ~2900fps velocities (but not in a Garand). Therefore the 2700fps load has less chamber pressure in the bigger .30-06 cartridge than the smaller 7.62 Nato cartridge, correct? So a lower pressure round would "kick" less than a higher pressure round, all other things equal. Right? Somehow it makes sense to me.
FWIW, I've only shot two .308 rifles, but own about 5 or 6 .30-06's. One was an M40-clone (w/Remy reciever) that shot the 168gr match loads. Felt about like my 03a3 Springfield. The other was an M40a1 clone with a Model 40 receiver that shot the heavier (180-190 gr load?) bullet. For a 13 lb gun it kicked like a flippn' mule!
But both were lazer beam accurate.