1 MOA is 1 MOA as far as the rifle is concerned -
Exactly. Alot of people misunderstand the term MOA and believe it means an inch no matter what distance you are shooting. As others have already said, its an inch (technically 1.047 inches per the official formula) at 100 yards. Then add an inch for every 100 yards thereafter. So to be exact, 1 MOA at 600 yards is 1.047 x 6 = 6.282" center-to-center spread.
Groups do often open up relative to MOA at longer ranges but it is usually due to either wind, the shooter, or the optics.