spo: the best ammo for the M14/M1A is US made mil-spec. That's what the rifle was designed for, if you are just going to bang away. If you are shooting in matches or are looking for greater accuracy and longer range capability, try the heavier mil-spec 173 or 177 gn ball theyu use for match or sniper work. As to who makes the better stuff - ? Most any US ammo maker who makes ammo to mil-spec is good to great.
Foreign made 7.62x51 should be marked as NATO or to NATO specs, which are the same as US specs. Some 7.61x51 ammo is not made to US specs, so be careful when buying foreign made ammo just because it isn't as good. JMTCW. Mikey.