Quoted: the M1A gas system is "self-regulating" in that it only uses what it needs - loads way out of the norm may be too much for it on a regular basis but i don't see how. perhaps someone with gunsmithing experience can elaborate.
overall, i would think any standard pressure hunting load 150 to 180 grain in the .308 should be fine.
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Though I am new to M1A, and the M1A is a self regulating due to the excess/unused gas vent hole at bottom of piston tube, I assume magnum loads being No-No cause the magnum load creates higher pressure hence faster velocity bullet, causing the more recoil force on bolt and extra speed on the op-rod, kind of like +p+ ammo on handguns cause more accelerated wear.
Who wants extra fatigue wear on your M1A when standard pressure ammo, high quality hunting bullets will do just fine.
ETA: I use magnum loads on my beefy bolt gun, but not too much also cause my standard velocity ammo has great results too.