How to Mount a Scope on a AR15

AR 15

If there was a rifle built to allow you to easily add a scope mounting system, it has to be without question the modern flat top AR-15.

Why is this the case? Because this rifle uses a standard fixture, the old WWI Picatinny rail or a modern offshoot of this system, the Weaver rail. This reduces the workload by 75% when taking on the job of mounting glass optics on this rifle.

Tools You Need

Getting ready to install the best ar15 optic includes finding the correct screwdrivers or hex head systems for the rings and bases. Then, within minutes, you can have the whole system set in place and tightened down.

If this sounds easy, it is! These sight receiver base builders understood the exact needs of the shooter for his or her sights on a specific rifle.

precision scope tuning kit

While I have made this seem as easy as apple pie, there are a few rules to follow that keep the ball rolling toward an easily competed project. 

Cantilever Scope Mount

Here I have some solutions to problems you might encounter when adding those rifle scope mounts.

While the anchor of the Weaver-style rail accommodates several different single screw clamp bases, the best base setup I recommend is the cantilever one-piece base and mount system. This is a stop and drop system that can’t be installed incorrectly.

how to properly mount a scope

The cantilever is a one-piece base and ring unit. There is no separation and no extra screws are used. In fact, the only screws used are those on the rings themselves.

how to mount a rifle scope on a picatinny rail

For the AR receiver, the screw claw mount onto the Weaver notched rail allows a good fit for the scope and adequate spacing between the rings.

Be sure you have adjusted the reticle to a position that makes sighting seem natural and lacks any undue distortion or stress.

Selecting bases and rings can be difficult. Some system designs have up to five or six separate parts that must align and come together to perfection. Nice idea, but not fun to assemble at all.

Again, the Cantilever makes all that work go away and sets up straight and true every time. This is because it can’t go any other way but correctly. 

Mounting The Scope & Leveling Up

mounting a scope on an ar

To easily install and adjust the scope, set the scope body onto the lower section of the rings then screw down the upper receiver. Only tighten enough to hold the scope in place. Do not screw down so tightly that the scope cannot be rotated.

Now, align the crosshairs. Due to the fact that I have been lining up scopes for the past 80 years and do this about once or twice every month around the shop, I trust my instincts to find a true alignment with the rifle's receiver.

Do this enough and you can get by just by using a sharp eye when taking on the task. 

For the inexperienced gunsmith, simply line up those crosshairs to a level point by setting the rifle on a solid rest to prevent it from moving around easily. You can check your accuracy with a bubble level.

how to mount a scope

Now, looking through the scope, select a horizontal line downrange and line up the scope's horizontal crosshair with that line.

When lined up, tighten down the screws one at a time by crossing over from one side of the rings to another. Be careful not to overtighten the screws as that may result in a broken thread. A broken thread is not an easy fix at all.

The correct screwdriver and bit system reduce the chance of damage to the rifle, mount, or screw surface. 

When all screws are tight, recheck your alignment. Now, one by one loosen each screw again and apply some non-permanent Loctite to the threads, snug the screws again, and recheck each screw as the threads will stretch. An additional half-turn could be required.

ar-15 with scope

When selecting a one-piece mount like this system or when mounting individual bases and rings, pay attention to the base height. Scope bell housing sizes and tube dimensions are very different with sizes ranging from 33mm to 50 mm.

The bell needs to clear the end of the rifles receiver or Weaver base system when screwed into place. Again, the one-piece cantilever systems offer ample height, but nothing is always a sure thing. Pay attention to what you're buying and how it all goes together. 

With your scope in place it is time to “zero” the whole package. If you are lacking any of the needed supplies or tools to complete the mounting process, check Amazon to find products at affordable prices.

Want to know how far can AR 15 Shoot accurately? check out our guide.

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