This is why the best long range shots in the word never use scopes. Oh... wait...I'm no expert, but barrel length will determine velocity, right? Allowing wind, atmosphere to disturb the shot. I've been told 22" is a perfect barrel length for the 7.62x51. Also, an M1a should do fine with iron sights out to 600, mine will. A 300 yard zero, then dial what you want, pretty simple. Scopes are nice,but not needed on the M1a. A set of Crager SS's look good on a car, but don't make it faster. Go nekked , go long. JMO.
Simple fact. The better you can see it, the easier it is to hit it.