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Co-Witness Sights With An Iron Sight – Absolute vs 1/3 Co-Witnessing

To co-witness sights means that you have an iron sight mounted in addition to your red dot or holographic sight. You typically use both targeting devices at the same time. Let’s check out how to co-witness sights in this article.

Using an iron sight to co-witness your red dot sight

One of the main advantages of co-witnessing is that you use the iron rear and front sight as a backup in case your holographic, reflex or red dot sight stops functioning. The abbreviation you might have seen is BUIS which is short for backup iron sightsWhat Are Iron Sights? How to Use Iron Sights and Shoot with them?

Co-Witnessing Definition

To co-witness sights started when the first red dot and later holographic sights were available and became a presence in the tactical and military markets. At that time officers and shooters required a backup sight in case their main, electronic-based one, would stop functioning or gets damaged. 

Reliability wasn’t as good as today and having a BUIS was a necessity from that perspective. Even today it makes sense to have a backup iron sight, specifically when in tactical or combat situations as there’s a chance that the main sight gets damaged or runs out of battery.

Both sights are lined up on the rifle and used at the same time to acquire a target. Therefore, even if the red dot gets damaged, there’s no need to install, zero, flip-up, etc. the iron rear and front sight as it is already in place.

You’ll find the combination of a red dot, reflex, or halo sight paired with an iron sight most often on any of the AR family of rifles. That’s not to say that you couldn’t or shouldn’t do it on any other kind of rifle.

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What is Co-Witnessing?

In shooting to co-witness refers to the ability to verify and acquire a target through lining up your holographic, red dot, or reflex sight with your iron sights. When everything is set up correctly, the dot of your electronic sight sits on the tip of the front sight of the iron sights setup.

After zeroing the setup, your tip of the front iron sight and your red dot will mark the target even in the dark. This does not take any bullet-drop and ballistic adjustments into consideration.

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Why do you need Co-Witnessing?

Red dot and holographic sights are great for quick target acquisition. When they initially started to be used in law enforcement and military applications, their reliability wasn’t that great.

To co-witness a sight became popular due to the likelihood of your electronic optics failing. These inherently had a risk of failing, especially during the early days of when these new sights were invented.

You can imagine that there can be a devastating outcome if you rely on your red dot and it fails while you are in a combat or self-defense situation. In a hunting or competition setting, it might only scratch your pride but otherwise, the loss of your primary targeting system can lead to serious implications.

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Therefore, co-witnessing became a new standard for combining an electronic sight, like a red dot or reflex, with iron sights that would be used if the main sight failed. A correct co witness setup allows you to switch from your optic to your iron sights seamlessly. You can continue shooting without having to manipulate your setup by for example replacing your main sight. 

Iron sights themselves are also a great targeting help for close quarters. Many shooters with a co-witness rely on the iron rear and front sight part for short-range shots and leverage the improved optics of their reflex or holo sight for medium distances. It becomes second nature if you train enough with such a setup.

A final advantage of using an iron rear and front sight in combination with your optical one is that the iron sight does not suffer from parallax influences. As the iron rear and front sight does not have any glass that could manipulate your image, parallax is ruled out which can make it easier in some situations when you shoot at small targets. 

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Co Witnessing a red dot sight

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How is Co-Witnessing done?

Setting up your Co-Witness

Setting up a co-witness sight is pretty easy. It’s three steps to get the setup completed:

  1. Get iron sights
  2. Zero your iron sights
  3. Mount your optics and adjust the setup

Let’s have a closer look at each of those steps to co-witness sights and what you need to consider.

Iron Sights Setup

First, you do need to have iron sights. If your rifle doesn’t come with them then you can get some and install them.

The red dot or holographic sights you can find today typically are designed for co-witnessing. Their mounts match the typical iron sights heights for AR’s.

If you have a sight that is not designed to co-witness or you use non-standard iron sights that are too high or too low then you will need risers or spacers to bring one or the other up to level.

Once the sights are installed you have to make sure that you can see them through the optic. When you install iron sights with the front sight on the gas block and the rear sight mounted on the flattop rail then you will usually be able to see right through your red dot or holo sight. This is true when you use a standard A4 upper receiver as many iron sights and red dots and holo sights like EOTech (Reviews of the Best EOTech Sights Knockoffs) or Aimpoint come aligned for such a setup.

You can find a variety of risers or spacers for nearly all sights to match them to the specific rifle you’re using. Depending on the rifle you will have to look for such risers to allow you to co-witness your iron sights with your holo or red dot. In case you’re wondering, a riser is simply a little platform that you attach to the rail which then allows raising the mounted sight (How do you Correctly Measure Scope Ring Height for any Rifle Optics?). You can get them in different heights and based on your rifle and sights you will have to measure and test to find the correct riser(s) to make your setup work and get all components lined up.

Zero your Iron Sights

After installing and lining up the iron and optic sights, you need to zero your iron sights. Start with zeroing your irons first without the optic. 

This makes sure that the front sight and back sight always stay on your rifle are zeroed (How to Zero a Rifle Scope at 100 Yards). If you switch your red dot or use it on multiple rifles then you always have the irons being zeroed so you can align the optics and have them set up correctly without having to fire any shots!

Make sure you zero your irons with a number of rounds to confirm zero. This way you can also rest assured that you’re not losing zero after shooting a few rounds.

Mount and adjust the optics

Once your irons are zeroed, you mount the optics. Adjust the height so the red dot aligns with your iron sights perfectly.

Flip your iron sights up and then you adjust your optic. You want the aiming point of your holo sight or red dot to match the one on the iron sights. You want to align the optics using the same setup that you used to zero your irons. That way you have the same reference points which help you align everything faster and more reliable.

You can and should fire a few rounds to confirm that everything is still zeroed. That’s all you need to set up your optics with iron sights for co-witnessing.

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Lower ⅓ or Absolute Co-Witness?

There are two common ways to do co-witnessing between iron and red dot/holographic sights. There are the absolute co-witness and the ⅓ variant.

Absolute Co-Witness

If you use fixed iron sights (BUIS), then an absolute co-witness has the advantage that everything is always lined up. If your red dot stops working then the irons are in the right position and you can continue aiming and shooting without any adjustments as you see the rear and front sight in your sight picture.

Setup of red dot and iron sights for absolute co-witnessing

The continuous alignment between the irons and the optics in the absolute co-witness is helpful in varying lighting situations. If you move with your red dot into a bright environment you might have your dot not bright enough for the changing light conditions and the iron sights can be tremendously helpful in that case.

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Lower 1/3 Co-Witness

Using a lower 1/3 co-witness sight setup compared to an absolute one is that you have less clutter and items in your sight picture when aiming. Looking through your optic you basically only see the reticle and image that the red dot or holo sight provides you. The rear and front sight posts are out of the way and you can use the optic to its full advantage.

Setup of red dot and iron sights for Lower 1/3rd co-witnessing

It often happens in an absolute co-witness setup that your eye will focus on the front sight of the irons instead of focusing on the target. This cowitness can be distracting and slow down target acquisition.

In a lower 1/3 co-witness setup the iron sights are visible at the bottom of your sight picture. Alternatively, you can also use a flip-up iron and absolute co-witness. In that case, you have the flip-up iron out of the way until you want to use it for aiming. However, this approach to having a co-witnessed setup is not as smooth as having an absolute co-witness or lower 1/3 co-witness system.

If you want to use the iron sights in a lower 1/3 co-witness then you have to drop your head slightly to align with the iron sights which are lower (as the name says) in this setup. The advantage of the lower 1/3 co-witness is that you use your electronic sight without disturbance until you need and want to switch to the irons.

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Can you co-Witness a scope?

A scope and iron sights cannot be used to co-witness sights. You might think that potentially a 1x optical scope might work but it still won’t. The issue is that no optical scope today has a true 1x magnification no matter what the marketing material says – Best Rifle Scope Magnification and Power Range.

Having any magnification screws up the alignment of the optics setup and makes it impossible to co-witness sights. Even if you were having a true 1x riflescope you would still run into issues due to the multiple lenses in the scope that refract light. This forces you to have to focus the image which in turn will ruin the iron sights setup.

Besides that, you also would face issues with eye relief for the scope. Let’s say you have a scope with 3.5′ eye relief. You have to have the back part of the iron sights between your eye and the scope. 3.5′ is not a lot of space for that to work while also making sure you won’t have the rear sight slam into your eye every time you shoot.

The last factor is that you have no need to have a backup for an optic-based riflescope. The reticle is typically etched into the glass and there’s no battery being used. There’s basically no situation where you would not be able to use the riflescope to aim.

Can you use iron sights with ACOG?

You certainly can use iron sights in combination with ACOG’s (find the best ACOG alternatives). But, you can’t use it in a co-witnessing setup

ACOG’s have magnification which makes it not feasible to use a combination of an ACOG with iron sights to co-witness.

You can mount your ACOG on a QD mount though. That enables you to flip it to the side and use your iron sights instead.

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Final Thoughts

It’s straightforward to set up your iron sights and your red dot or holographic sight to co-witness sights. Whether you want absolute co-witness or lower 1/3 co-witness, you do need to get the correct height when you mount your red dot optics. Once that’s correctly determined then it’s straightforward to set up the co-witness on your rifle. If you follow the outlined steps you’ll get a solid setup that allows you to continue to shoot even if your illuminated dot vanishes.

There shouldn’t be a need to remove your iron sights so just leave them in place and zeroed. Even if you switch the optics and leave the iron sights in place, you’ll be able to align the whole setup easily and quickly.

One thing to remember is also that you have to occasionally train yourself to shoot only with the iron sights. What good does it do to have them as your backup and you can’t shoot with them?

Using an iron sight to co-witness your red dot sight
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Read more about iron sights, holographic sights, and red dots for hunting or shooting!

Aaron Bennett