Motega, it sounds like you don't really understand the concept of the "cone of dispersion" of a group, and how MOA are used to measure it. If you can shoot a 1" group at 100 yards, it doesn't mean you will also shoot a 1" group at 300 yards, even if you take all external ballistics like wind out of the equation. Your groups expand at a uniform rate as the range increases, producing a cone extending out into space within which your bullets impact. Basically, range magnifies any tinly little variation in rifle movement or bullet flight path. A tiny fraction of an inch movement of your muzzle can result in you missing your point of aim by several feet at 100 yards, and you will miss by twice as many feet at 200 yards. That is because you are changing the ANGLE that your barrel is pointed toward the target, and the distance between the rays of an angle increase as distance from the vertex increases. So when shooters are trying to measure their accuracy, they are really trying to measure the angle of the cone of dispersion within which their bullets strike. That's where Minutes of Angle come in. A minute of angle is 1/60 of a degree. It is almost exactly 1" per 100 yards.
This is much better explained with diagrams and such. Come to an Appleseed if you want to get a better explaination of it, and of how to use it to sight in a rifle at any range with just one adjustment. You will also learn a lot about the fundamentals of rifle marksmanship.
About the ACOG... A FAL with an ACOG would be a great battle rifle, and could easily be relied on to put down 2-legged varmints out to 600 yards or more. The ACOG reticle is designed for quick hits on man-sized targets at unknown distances. They have exceptionally clear glass and are tough as nails. But they aren't particularly high magnification, and their reticles aren't really designed for precision. Not to say you can't shoot some very precise groups with an ACOG... my ACOG'ed M1A has put up some 1 MOA groups with surplus ammo... but they aren't really optimized for precision. Kind of like how a 24x target scope wouldn't be optimized for combat from close quarters out to 600 yards, like an ACOG is.