Don't know how many clicks(MOA) your scope has but lets asume it has 80 MOA or 320 clicks. When you mount the scope with regular mounts, generally you will be at "O" in the middle of the range and will have about 160 clicks up and 160 clicks down. Now the problem is that those 160 clicks down are wasted! You don't go below "O" when shooting. You do go up, about 22 - 24 MOA , for 1000 yards. When you want to shoot further, much farther say a mile , you may need more than 160 clicks. The 20 MOA rail or mounts raise the back of the scope and give you an additional 80 clicks of up. You lose 80 clicks of down but you weren't using it anyway. If you added 40 MOA you wouldn't have any down and all up. All scopes are different and some may work out to be above or below center with regular mounts. After mounting a scope and getting it sighted in, turn it up, counting clicks, and see what you have. I also count full range before mounting scope so I know how much there is to play with when I'm done. Some 1000 yard shooters like to have the scope set up so that 1000 yard adjustment falls right in the middle . Don't know if this helps accuracy or not . Hope this helps!!