Like others stated many factors come into play. MOA obviously is proportional. If it wasn't dialing the proper MOA to your long distance target wouldn't work. As far as groups it is a different story. If you have a good rifle the groups will actually shrink proportionaly as the range increases to a certain point where velocity spreads, etc factor in. When a bullet leaves the muzzle it travels in an eliptical orbit around the centerline of your bore. To visualize this understand your rifle is not throwing a bullet left, then right, then low and then high of the others to produce a group. Your group is the bullets printing at different points along this elliptical orbit around the center of your bore. When it hits the paper it will always be somewhere within this predictable orbit. That is your group size if you take out shooter error and a few other variables. Now this orbit will shrink as the range increases to the point this orbit will become proportionally very small in relation to distance. That is why my baby 338-378 rarely shoots under 1" groups at 100 yards but will shoot 7.5" at 1000 yards. Considering velocity spreads and operator error that means the bullet is stabilizing around the centerline of the bore.
Unfortunately as the distance to target increase and time of flight increases as your bullet slows exposing it to the elements of atmospheric conditions and gravity along with velocity spreads causing bullets to drop at different rates your groups begin to spread disproportionately. Then take into effect spin drift, correolis effect, case and bullet differences in weight and capacity, BC not exact, angle to target, etc, etc and you begin to wonder how in the hell did I make that shot at a mile.
These are the reasons I draw the line between long range shooting and long range hunting. Animals are very precious to me and I have deep respect for them I guess coming from my Indian heritage. So I draw the line on shooting at something to see if I can hit it versus shooting at something I am 99% sure I will kill it. That range varies with conditions but rarely exceeds 1000 yards. That is why most of my rifles are set up with components making them the best they can be to a half mile because usually I can stalk to within that. Sometimes I can't and this year hit some 900-1100 yard one shot kills when the conditions allowed me to be certain of those shots.
