I could be wrong, but I believe this is pretty accurate. MOA stands for Minute Of Angle. Angles are measured in degrees, and degrees can be divided up into 60 minutes. Minutes can be divided up into 60 seconds. At approximately 100 yds, the distance covered by a 1 minute arc is about 1 inch. So when someone say 1 MOA, they mean 1 inch groups at 100 yds.

Sometimes I hear people say "MOA at 200yds". What the really means is 2 inch groups at 200 yds, but what they are probably trying to say is 1 inch groups at 200yds....which is actually 1/2 MOA. I am trying to do the math (trigonometry) right now, but it has been a while, so I am struggling....are there any smart trigonometry people out there who can confirm this?

Hope I helped.