How to mount a scope on an AR-15

April 11, 2021

How to mount a scope on an AR-15

How to Mount an AR-15 Scope

If you’d like a brief introduction or refresher course on how to mount a rifle scope on an AR-15, you’ve come to the right place. The experts at Warne Scope Mounts have been designing American-made rifle scopes and other firearm accessories for decades. This means we know a thing or two about how to properly mount an MSR scope – and prevent performance-hindering scope slippage or misalignment.

Scope Mounting Tools

Regardless of if you’re working with one-piece or ring mounts, you’ll need a few tools to fit your scope to your AR-15. If you don’t have them already, these are the tools you need to get:

  • A torque wrench for attaching your mount to your rifle.
  • A hex key (that came with your scope and mount) or precision screwdriver bits for attaching the base and rings.
  • Optional Small bubble level(s) to align the scope, ensuring its accuracy.
  • A gun rest or workstation with plenty of space for using bubble levels to double-check your scope’s precision placement on your rifle.
  • Rubbing alcohol for cleaning different scope components (rail, rings, screws, etc.) while installing your mount and scope.

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Mounting a Rifle Scope on an AR-15 Using a One-Piece Scope Mount. 

If you are using a two-ring set, you would mount the scope as you would on a Picatinny railed base. If you are mounting, you scope on an AR15 MSR using a one-piece scope mount it's a little different because you will be able to move the scope within the base and the base within the slots. Some factors to consider is mounting area on your scope tube and eye relief and being able to reach under the scopes eyepiece to reach the charging handle with your forefingers. 

Once you’ve purchased your correctly sized one-piece (cantilever or non-cantilevered) or ring mounts, gather your tools at your workstation/shooting rest. Here are the basic steps you’ll need to follow to mount your scope to your AR-15:

Note final torquing should be done at the final step. 

1. Rifle Safety ­- Ensure your AR-15 is empty. Removed the magazine. Make sure the chamber is empty. Double-check everything.

2 Shooting Rest – Secure your rifle firmly onto your shooting rest or gun vise. The last thing you want is for it to wiggle around while you’re working on it. Check levels with your bubble level.

3. Cleaning – Use rubbing alcohol and gun cleaning patches to wipe oil and grease off the mount, rail, screws and other mounting components.

4. Placing the Mount – Carefully set the mount atop the upper receiver. Some shooters will put as far front on the upper receiver rail as they can while others will drop it back a few slots. Do not to place it on the free-floating rail. You want to avoid adding excess stress to the rifle’s tube. 

5. Tighten the Scope Mount – First, hand-tighten the scope mount do not torque it down until you are certain that is in the slots you want it to be in. 

6. Adding the Scope ­- Verify that your rifle is level in a gun vice or at your workstation. Place the optic of your MSR rifle on the mount and secure the caps over the scope, tightening them with an “X” pattern – but don’t lock them down completely. Make sure there’s a small gap on each side of the ring for positioning the optic. You can snug them down on an expired gift on each side to get an even ring gap. The scope should still be moveable with a little effort but not too loose that it moves when tilted. Now you can adjust the scope for  proper scope eye-relief. Once you’re happy with the distance between your eye and the scope, level the reticle. 

7. Leveling ­- Check once again that your AR-15 is level in your gun rest. Then verify the reticle alignment and adjust if necessary. Use the instructions on your leveling tool. If you do not have a leveling tool and do not plan on shooting long range precision, you can level it to your cheek weld which is preferred method for those who shoot standing up like hunters and 3-gun shooters. This can be done by hanging a weight from a string and shouldering the rifle and aligning the vertical crosshair with the string. Once the reticle is level you can torque it down. 

8. Torque Torquing the mount to the base requires the mount to be pushed forward in the slots and then torque to base to manufacturers torque specs. Then torque the ring caps to scope manufacturers torque specs. Never use any kind of thread locker on scope caps because this will change the torque specs from dry thread to wet thread and possibly damage your scope. Understanding Torque.

One last time, check the levels and reticle alignment. If everything is still level and accurate, give yourself a big pat on the back – because you have successfully mounted a scope on an AR-15 rifle. Now you are ready to  boresight your rifle.

Questions About Mounting a Scope on an AR-15 Style Rifle. 

Can I mount a one-piece cantilever mount backwards? Yes, you can as long as it is not a 20MOA mount. Keep this in mind though the cantilever scope mount was designed to push the scope forward so that the shooter could get perfect eye relief. If you are reversing that make sure you tried it facing forward first and that you are doing it for the right reason. I have seen shooters do it this way and that was because of the scope they purchased wouldn't work the other way with the way they held their gun, and it worked for them.

Can I use regular scope rings instead of a one-piece mount on my AR15? The problems you will encounter here is you will need ultra-high rings and you may have problems getting the scope out far enough to get proper eye relief and be able to reach the charging handle. But yes it is possible. 

What if I don't have a torque wrench to torque my scope mount? It is best to use a torque wrench however the biggest key is getting good even torque and not to over torque. By using the provided hex wrench snug the screws then give them about a quarter to half turn each it should be right at the point where it starts to get hard to turn with your fingers. If you have a lightweight scope requiring less than 20-inch pounds of torque, then a torque wrench may be required to prevent damaging the scope tube.