Ok I have been trying for hours to figure out the difference between a flat/15 moa/20 moa base etc. For my Mueller scope I ordered with 60 MOA internal adjustment it is said that on my 308 to reach 1,000 yds I would need a 20 moa base. And that is a darn shame as I didnt want to have to spend more money now. But I dont want to have redo all my sighting/dope in a month when I upgrade. So I might as well do it now before I even start. In short I finally found my answer as to the different bases.