Riflescope mounts are a large part of our business and consequently we sell a lot of Picatinny rails. We offer these in various MOA and have always been surprised how many 20 MOA rails we sell. Over time it has become clear that many who buy these rails don’t actually know what MOA is and I have had many telephone conversations with customers who find that their rifle does not shoot as expected after mounting the new rail.
For those who do not know what MOA is, here is a short, and hopefully clear, explanation.
MOA stands for Minute of Angle. One minute of angle is 1.047 inches at 100 yards, at two hundred yards double that, 2.094″ and at 500 yards, five times the 100 yard figure or 5.235″. The adjustment on many riflescopes is calibrated in MOA and typically the dial will read “1 click = ¼ MOA at 100 yards”. That means each click equals 0.26175″ at 100 yds. Note that some scopes designed for target work may be calibrated at ⅛ MOA with each click being equal to 0.13” at 100 yds.
As you will have noticed, 1 MOA is very close to 1″ and for most purposes that is close enough. Some scopes are actually marked “1 click = ¼” at 100 yds”. Common practice is to take one click as ¼” at 100 yards. So at 200 yds I click is ½” and 500 yds 1.25″.
A Picatinny rail with built-in MOA is raised at the rear so that the scope points slightly downwards, thereby raising the muzzle. As each minute of angle is 4 clicks on your scope, a 20 MOA rail is 80 clicks at 100 yards. So your scope has gained about 20″ (20.94″ to be exact) in elevation adjustment at that distance. At 500 yards 20 MOA raises elevation by a whopping 104.7 inches, almost 9 feet.
A rail with 10 MOA is half that, 10.47” at 100 yds and a 30 MOA rail 1.5 times, 31.41” at 100 yds.
Rails are made with MOA built in because as range increases you may find you run out of elevation adjustment, or at least are running very close to the upward limit. Choosing a rail with built in MOA solves this problem but remember the old adage that there is no such thing as a free lunch. Whilst you may gain 80 clicks in height adjustment you loose 80 clicks in the downward direction. At long range this is not a issue but it may be at closer range when you find you are shooting high.
If the maximum range you shoot at is less than 200 yards you do not need a rail with 20 MOA. In fact, depending on calibre, you probably don’t need it until you pass 500 yards. At distances between 300 and 500, 10 MOA can be useful.
Before choosing a rail with MOA adjustment consider the range you will likely shoot at and the trajectory of your rifle calibre.