How To Mount Scope On AR 15 With Front Sight

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A front sight does not physically prevent a scope or optic from being mounted to the rifle. If you have a flip-up front sight and are not trying to co-witness with iron sights, then just flip the front sight down and install the scope normally. If you have a fixed front sight post (FSP), then you might have some more difficulty.

A non flip-up FSP will obstruct the view of a low magnification scope on an AR 15, and if you don’t properly co-witness a red dot or other non-magnified optic with your irons, it will be distracting and difficult to shoot with. The FSP won’t be visible at higher magnifications so your shooting should be just as accurate, on target, and your image as clear and light as it would be without it.

Steps To Mount Scope On AR 15 With Front Sight

Decide Whether You Want Absolute Or ⅓ Co-Witness

how to mount scope on ar 15 with front sight

In other words, do you want the rear and front irons to align perfectly with the red dot so they all three match up when you have the irons flipped up? Or do you want the front and rear sight to utilize the lower ⅓ of the view through the red dot? If you’ve never shot with co-witnessed sights before it’s hard to know the answer to that question. Here’s my best shot at answering it:

I find if you have flip-up irons, then absolute co-witness works great because you’re only using the irons in case the red dot has an issue. If don’t have flip-up irons, or the common combo of one flip-up and the other on your ar 15 not, then I would recommend a ⅓ co-witness.

The reasoning is simple: when I’m using a red dot, the front sight just gets in the way of the reticle, but I also like to have my red dot a little lower to the rifle if possible.

Most AR-15s use standard heights for their iron sights, and red dots will usually come with a riser height that is designed for either absolute or ⅓ cowitness. 1-inch risers will give you ⅓ cowitness while .83 inch risers will usually give you absolute cowitness.

Scope rings aren’t nearly as standard in size or to mount, but with a little research you can find short, medium, and tall rings to match the same gap level to get where you want the FSP to appear in your sight picture. Mounting isn’t complicated, but to get the position right for shooting on your AR 15 can be difficult. Rest the gun on something to hold its position while you use your tools.

Remove The Sights, If Desired

how to mount a scope

The rear iron can be fairly straightforward to do this with, but you need to be careful with the front sight because it may have the gas block in it. If it does, if it were me I would just take it to a gunsmith and tell them what I wanted. Some things just aren’t worth the risk of doing wrong.

The rear iron on the upper receiver of the ar 15 rifle should be easy to remove. Just unscrew it from the flat part of the upper receiver. An AR 15 with front sight can be more complicated though. A crucial step is determining if the FSP is combined with the gas block of the gun. It’s just a matter of googling the rifle or rifles you have to see.

You can also consider swapping for a free floating handguard, which also improves accuracy and makes the process easier. Keep reading for the next step and the rest of the steps to get your ar 15 rifles ready for shooting.

Attach The Scope Mount To The Rail Mount

mounting scope on ar 15 with front sight

How this is done depends on what kind of scope mount you have and what kind of scope, but assuming you have a red dot, then the optic and scope mount may come attached and ready to just clamp right onto the rail. In most cases, you want to use a bubble level to get the ar 15 aligned vertically with the scope. That bubble level will come in handy a few times.

If you have a scope and two rings, then there’s a bit more to it but your goal is simply to get them spaced where the scope can sit at a comfortable distance away from your eye. When you mount a scope, you may want a torque wrench to get the screws to a specific torque level. If you over tighten you can cause damage during the process, but a hex bit tool can work too.

Most scopes can fit on the small picatinny that most AR weapon systems will come with, but you can go through all the steps to installing a free floating rail if that will have a positive effect on your scopes’ position and make things easier when trying to shoot accurately.

Attach The Scope To The Scope Mount

mounting scope on ar 15 with iron sights

Again, if you’re using a red dot, this may all be one step, but if not, once you have the riser plate or mount in place, just attach the scope to it per its instructions. I’d recommend a torque wrench here as well to get the sight completely attached but not too tight. The rest is simple, make sure there’s surface to surface contact around the entire scope, and avoid placing it too close to the barrel.

If using rings, you mount a scope by putting it in the lower rings then putting the upper rings on top of it. If you’re co-witnessing, check where the reticle is in relation to the irons.

Tips

Here’s the deal with magnification: if you’re at a low power like 2x-4x, then the FSP is going to obscure your sight picture to some degree. By all reports, if your magnification is higher, especially in the 9x+ range, the obstruction of the front sight fades almost completely and is not even noticeable.

When I first wanted to put a scope on my M&P 15, I ended up wanting to swap out the drop-in handguard for a free floating rail so I could mount a scope the way I wanted to, and that’s something I recommend everyone do at some point.

Use a bubble level when you mount your scope on your rifle. Look at your target through the scope during the mounting process to ensure accuracy with the weapon at that distance. Clean the surface with rubbing alcohol. Tighten, but don’t tighten too much. When you adjust after installing, do so gradually.

Final Thoughts

You can buy a scope mount or scope rings that get the scope as high or low as you need it on your AR15. You don’t need many tools beyond the screws to secure the scope mount or scope rings and the scope itself, a tool to tighten the screws, and a bubble level. All this is available for a reasonable price.

The mounting scope process is not as difficult as it may seem with the right tools, but you’ll want to avoid mounting your scope where it’s not secure on the gun or properly leveled. A scope or optic can help you be more accurate when shooting your gun at the range and while hunting. For a short but helpful video on this topic, check out this from Caleb Downing on YouTube:

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