i think a few people are a bit confused here.
MOA is a minute of angle, that's 1/60th of a degree.
FLEA, the MOA is fixed, it doesn't change (so long as the elements don't start interfering) with range. 1 MOA deviation at 100m is 1 MOA devation at 1000m (again, assuming the accuracy doesn't degrade, of course it does in real life because of the elements but that's not a fair comparison to make between rifles...this is why the ranges are limited like mele said to 100m or so). I'm really reitterating what mele said.
The MOA accuracy is found by trigonometry, 60*atan(group size/range).
So just to clear this up, MOA is not the same thing as group size, it's a standardised measure of what angle range any bullet will fly through (think of a cone from your barrel).