Look below your iron sights. Use the ammo stamped on the barrel. My guess, if its an M1A, will say 7.62. Always use whats stamped on the receiver. Of any rifle. Or handgun. American Eagle sells ammo with a label 'for use with M1A'. Its 7.62x51. Scope that thing(Basset makes a good mount but you will wait a bit). With a good scope, steady hand, good ammo and good eyes you'll knock 'em down at 700 yards.