If you are asking this question, you probably need a zero MOA rail. Zero MOA rails are completely flat, meaning that once they are installed on your rifle, they do not add any additional elevation to any mounted optical device. If you were to draw an imaginary straight line through the rifle bore to a bullseye on a target, and an imaginary straight line along the top of the mounted zero MOA picatinny rail, the two lines would would never meet, and the line from the rail would impact the target just above the bullseye.
Now imagine that zero MOA rail has a scope installed and there is an imaginary line running through the crosshairs of that scope to the target. If that scope is zeroed, the line should hit the crosshair at the same point as the line coming from the bore. Note that the lines are not ballistically accurate, they are just there to illustrate the point.
Many hunters will get their scope and rifle zeroed, put the caps on the scope turrets, and they will never again think of their rifle ballistics. This is fine, because a lot of hunters simply don't shoot at anything that is farther away than the range that their rifle is sighted in for. In the thick wooded mountains of Pennsylvania, we often don't get shots over 100 yards, so it's pretty common to sight your rifle at 100 or even 50 yards, and forget about it.
But there are a lot of hunters and shooters who regularly push the limits of the range of their rifles and optics. While their rifle may be sighted in at 200 yards for example, they might want to try taking shots at 400, 500, or even 600 yards. The longer the shot, the more elevation travel you need from your scope. Long range shooters often zero their scopes at a few hundred yards, but when they need to dial in a shot at longer range, they will sometimes run out of elevation travel before the scope can be dialed in. This is where a Picatinny rail with built-in MOA becomes necessary.
Let's go back to the imaginary lines in the rifle bore and along the top of the Picatinny rail. A 20 MOA Picatinny rail is tilted slightly forward, meaning the rear of the mount is a little higher than the front. This adds 20 MOA of elevation, which means the imaginary line along the top of the rail will end 20 MOA below the line coming from the rifle bore, and at a distance of 100 yards, 20 MOA is equal to 20 inches.
Once again, the lines are just for illustration and are not ballistically accurate. But this does illustrate the point.
The additional 20 MOA of elevation means that during long range shooting, the shooter has an additional 20 MOA of height to work with when adjusting their scope's elevation. This is usually enough elevation to make the shot, but there are some limited situations at extreme distances where a 30 MOA or even 40 MOA rail is necessary to prevent the scope from running out of elevation adjustment. If you aren't sure if you need a rail with more than 20 MOA, and you aren't pushing the elevation limits of your scope regularly, the answer is probably no.
Frequently Asked Questions
Which Picatinny rail do I need for my rifle?
This is a very common question, and it depends on make and model of your rifle, as well as other factors. Which Picatinny rail do I need for my rifle?
The Picatinny rail I want to buy doesn't say if it's 20 MOA. How many MOA is this rail?
If the rail you are looking at does not indicate MOA, it is a zero MOA rail. All rail that we sell with built-in elevation will specify 20 MOA, 30 MOA, or 40 MOA in the item title or description.
I don't shoot at long range, can I get a 20 MOA rail anyway?
Most likely, but it's completely unnecessary and can cause difficulty when sighting in your rifle. Since a 20 MOA rail naturally points the scope lower on the target at 100 yards than a zero MOA rail, you will have to dial your scope up quite a bit to get on target. If you aren't completely aware of this when starting, you may think something is wrong with your scope or rifle, because without any adjustment, you're sending shots almost two feet above the bullseye at 100 yards.
Can I start with a zero MOA rail and upgrade to a 20 MOA rail when I'm ready?
Absolutely. But any time you change the mount on your rifle, you will have to re-zero. Picatinny rails are not quick detach or quickly interchangeable. If you have a zero MOA Picatinny rail, consistently shoot over 400 yards, and want to push your shooting 500 yards or more on a regular basis, consider swapping to a 20 MOA rail.
How do I sight in my rifle scope?
You blasted through a whole box of expensive ammo at the range, and your scope still isn't sighted in. You're starting to doubt your Picatinny mount, your scope, or even your whole rifle. We recommend starting with a quality boresight. This will get your rifle on paper, so you burn less ammo getting dialed in. Check out our guide to sighting in your rifle with the SiteLite Ultra Mag SL-500 Laser Boresight: How to boresight a scope with the SiteLite SL-500