Scope base basics: 0 MOA or 20 MOA?

Custom rifles are ever popular as interest in both long range hunting and precision rifle series (PRS) matches continues to grow in 2021. For the novice shooter or anyone looking for a starting point in the shooting sports, navigating the terminology can be dizzying. From selecting a chambering to barrel twist rates, profiles, and lengths, the options for a modern rifle are endless. 

Acronyms like MOA, MilRad, COAL, FPS, MV, SD, ES, and many others further add to the confusion. To make this information less confusing, more digestible, and more searchable, we’re going to address these topics individually so it's easier for you when you're choosing a new rifle scope. 

As an optics retailer, one question we encounter frequently regards the selection of a scope base and/or rings. There are generally two ways to mount an optic to a rifle. One method uses a pair of rings that attach the scope directly to the receiver. 

Another option uses two separate components: a picatinny rail scope base that attaches to the receiver, and either a single-piece mount or a set of scope rings that attach to the picatinny base. This article concerns the later method.

Whether you’re purchasing a custom receiver or mounting a scope base to an existing action, choosing the correct base for your needs is important. The two primary variants you’ll encounter are the 0 MOA and 20 MOA bases. 

The difference between the two is quite simple. The number represents the cant (think tilt) of your scope base. With a 0 MOA base, there is no cant to the base and it will sit level to the action. A 20 MOA base has a slightly negative cant that starts higher in the rear and ends lower in the front of the base. 

If you’re wondering what MOA is, what cant is, and why you might need this, you’re about to find out.

What is a minute of angle?

A Minute of Angle (MOA) is simply a measurement of angle that most rifle scopes use as a standard metric of adjustment in windage and elevation turrets. The 20 MOA base points your rifle scope downward in elevation by 20 minutes of angle. By angling the scope down, the barrel must be raised up by 20 MOA compared to a 0 MOA base in order for the reticle to be pointed at the exact same point. 

This may be desirable for a number of reasons, but probably the most common reason to opt for a 20 MOA base is because of the limited range of elevation adjustment across different rifle scopes.

Let’s say for example, you have a rifle chambered in 6.5 creedmoor that needs a 100 yard zero. If the rifle scope is mounted on a 0 MOA base, a greater portion of the rifle scope’s usable range of elevation adjustment will be used to bring the reticle up to zero height than would be used if the same scope was mounted on a 20 MOA base. 

Since most shooters don’t dial adjustments under their zero distance, the goal is to make most of the adjustment range on a rifle scope dial up to shoot out at targets further away. Ideally, if the rifle scope has 60 MOA of elevation adjustment, 55-60 MOA will be available to dial up. Mounting a scope to a 20 MOA base makes that more attainable. That said, it’s not necessary for every rifle.

If your goal with the 6.5 creedmoor mentioned above is target shooting out to 600 yards or 1000 yards and beyond, then a 20 MOA base is essential. On the other hand, if you’re looking to hunt deer at 400 yards and closer, you likely won’t use your scope’s full range of elevation adjustment. 

Long range shooting is really the greatest factor in why you may need to consider a 20 MOA base. While everyone has their own definition of “long range”, typically, anything from 500-600 yards and beyond could fit the description.