Get some of the Freedom Munitions ammo. It's about as inexpensive as anything out there right now and is pretty good shooting out of a few 308 platforms I've fed it through. Have not grouped it out of an M1A though. If you can afford it, find some M118 / LC LR and try those out.
Funny this topic should come up.
Been wanting to tell folks that I saw Federal white box 7.62x51 NATO at Dicks for 18 bucks a box (Federal .243 Win is 23 a box).
I don't own anything in that caliber, but it seemed like one hell of a bargain.
Just thought I'd pass it along.
Makes me want to buy a crap load of it so I have an excuse to buy a new rifle.
Is 308 and 7.62 NATO the exact same identical and interchangeable round?
If you choose to use .308 and not 7.62mm NATO ammo be careful as the primers are different and a possible slam-fire could occur.
I use 7.62 Federal XM80 surplus as blasting ammo. Works well. It shows every now and then as loose 200 round ammo cans. Also been using PMC bronze 308. This is in a NM.
ETA: I also heard of some South African ammo that had issues. Not sure about though. Might have been corrosive. I generally would stay away from anything suspect, especially in a M1A. They are too nice to chance it. BTW, be prepare to draw some attention. Mine always does at the AGC.