This post has been a long time coming. Ever since I wrote my first guide to AR-15 optics, I knew I was going to have to tackle the topic of optical cowitness. Somehow, the wherewithal to sit down and write it evaded me. Well, no more.
Years ago, when this topic first became a “thing” to take seriously, the phrase really only applied to rifles. Today, with the prevalence of pistols equipped with optics, it’s even more relevant.
It used to be an assumption that any rifle equipped with optics would also have iron sights. It was strictly necessary in the event your primary optic failed you. Today, I don’t think this is true anymore, and my stance is that it’s good to have back up sights as long as they make sense and don’t detract from your primary optic.
What do I mean by detract? Think of instances like my Elcan or a classic TA-31 ACOG. For me, using those optics effectively means moving them far enough to the rear of the receiver that there’s no room for back up sight. Moving the sight forward would lead to compromising my shooting position.
I would rather not compromise my primary sighting system based on the possibility of employing my backup system. Anyway, I’m getting off track, because as much as I love Elcans and ACOGs, they aren’t relevant to a discussion about cowitness.
Defining Optical Cowitness
With cowitness, we’re talking about configuring your rifle so that you can use two different sighting mechanisms simultaneously. In other words, you can use your red dot or holographic optics while keeping your iron sights usable. This works because light still passes through the optic regardless of whether it’s working or not.
Why would you want to do this? There are a few schools of thought, but they basically boil down to redundancy. Optical devices can and do sometimes fail, even the best. It could be as simple as the battery running out of juice, or maybe the optic took a hit hard enough that the electronics simply stopped working. In either case, the capability to instantly switch to your iron sights means you’ve lost no time.
You need to understand a few limitations, though. First, if the lenses of the optic are damaged or obscured, then your cowitness is also not going to work. You must still be able to see the front sight in order to aim. I’ve seen this cause problems with fogged up lenses, damaged/cracked lenses, mud, or distortions caused from rain.
Secondly, optic cowitness only works with red dot or holographic optics. Anything with lenses that bend light, such as low-powered variable (LPVO) or prismatic scopes, will not work. Even if those scopes say they are “True 1x” magnification, the light still bends through the lenses and therefore cowitness doesn’t work.
AR-15 Cowitness Heights
Hang around enthusiasts long enough and read discussions about cowitness, you’ll inevitably come across discussions comparing absolute cowitness and lower third cowitness. These are two ways of describing how high the optic sits relative to your iron sights.
The naming refers to where the iron sights appear within the optic’s window. Here are two examples where I’ve drawn a line at about the optic’s centerline, and you can see how this relationship works.
With an absolute cowitness configuration, as pictured with the EOtech above, the center of the optic lines with the height of the iron sights. For an AR-15, we generally assume that the iron sight plane has a 2.6″ height over bore (HOB). That’s measured from the center of the barrel’s bore to the tip of the front sight.
When we employ an absolute cowitness height, we want to mount the optic so that the center of the viewing area sits right at that 2.6″ HOB. Every optic and mount combination is different to achieve this. So the mount height to get an Aimpoint PRO to absolute cowitness may be different than something else.
Assuming you use the same zero for both iron sights and the red dot sight, you should expect the center of the dot to perch almost exactly on the front sight post of the rifle.
Advantages to Absolute Cowitness
For me, there are three advantages for an absolute cowitness height with your red dot.
Firstly, you maintain the same solid cheek weld you’ve grown accustomed to with your iron sights. In effect, mounting a red dot this way means you don’t really have to change anything about your shooting position, but now you have the benefits of acquisition speed and parallax red dots bring to the table.
Secondly, there is a nice level of redundancy should the optic fail. Since the front sight appears in the middle of the optic, you can use the optic itself as a large ghost ring rear sight in a pinch. Of course, this only applies to situations where your rear sight is not available for some reason.
The third advantage is consistency around what “absolute” means. Since the HOB for AR-15 iron sights is a known number, manufacturers all target that height for their products. The net result is that you have a much easier time doing “mix and match” with components like red dot sights and magnifiers from different brands.
Disadvantages of Absolute Cowitness
When it comes to disadvantages, there are two that come to mind.
First, if you’re using a fixed front sight as I tend to do, then you might find the tower in the field of view to be distracting. Some people found that when the center dot and front sight tip align, they unnecessarily slow down their target acquisition trying to get things lined up just right.
With an arrangement like this, it’s easy to forget that the optic and sights are separate instruments and operate independently of one another. The red dot is the primary tool, and as long as you can see it and put it on the target- you’ll hit it. The iron sights only become a factor if the primary optic fails. I’ve illustrated this in some of the photos below, where you can see the reticle doing its job even though the iron sights aren’t aligned.
I’ve also found that I don’t like looking through the rear sight in order to look at the dot. When I’ve mounted absolute cowitness sights on a rifle with fixed front and rear sights, like the Minuteman Rifle, I wanted to look over the rear sight. That made me push the red dot to the upper portion of the window. In the above photos, you’ll see a representation of how that looks.
Both of these issues are solved with folding sights.
The second major issue with absolute cowitness stems from bulky accessories on the front of the rifle. If you have a PEQ, DBAL, or similar mounted on the top of the handguard, there’s a real chance that it will be distracting to or even block your sight picture.
On that topic, absolute height doesn’t work if you’re also running night vision.
Best Use Case for Absolute Height
So who is absolute height best for? In my opinion, it’s the best “all around” height if you fall into one or more of these:
- You have a folding rear sight, and don’t mind the front sight post being aligned to the dot in the middle of the window
- The rifle has folding front and rear sights
- You aren’t running night vision or bulky accessories designed for night vision use on the top of the rifle
- You prefer a “normal” cheek weld to the rifle stock
Lower Third Height
Some years ago, the limitations of cluttered sight pictures found with absolute cowitness drove requests to “fix it.” Mount manufacturers figured the best course of action was raising the optic up slightly so that the dot now appeared over the front sight.
The actual difference very small, just a bit over 1/10 of an inch. The name lower third cowitness, often written as lower 1/3, comes from the way the iron sights now occupy the lower portion of the optic’s field of view.
This is is often less distracting, especially with certain combinations of iron sights. It also has a few other advantages.
One of the main issues with lower third cowitness is that there isn’t a single industry standard for what that means. Scalarworks, for example, uses a mount height of 1.57″ measured from the top of the rail to the center of the optic. Geissele uses that same height, but Larue Tactical uses a 1.64″ height while ADM places the optic at 1.69″ over the rail.
Advantages of Lower Third Cowitness
The primary advantage is reduced clutter of the sight picture. This is especially true if you are running fixed front and rear sights. In this configuration, you can still look over the rear sight and have the dot in the center of the window.
If you’re running folding sights, then this isn’t an issue to begin with so it’s also not really an advantage. That said, there are several schools of thought out there that dictate keeping the folding sights up at all times anyway, because when need to make the shot, then taking time to deploy folding sights in an emergency is problematic.
With a lower third cowitness, your cheek weld is also slightly raised. Some people see this as a secondary advantage, because it puts them in a more “heads up” position and don’t get as easily sucked into the optic.
Personally, I understand where this comes from- but I don’t think the lower third height is “heads up” enough to really give that advantage compared to something like a 1.93″ height (I’ll touch on this option in a bit).
Lower Third Cowitness Disadvantages
Most of the disadvantages of the lower third height are subjective. One person might look at the slightly higher cheek weld a compromised shooting position, while another could see the “heads up” position as beneficial.
To me, the primary disadvantage of this height is that it isn’t always consistent from manufacturer to manufacturer. As I mentioned before, there is no standard for what comprises “lower third.”
In practical terms, that means you could run into alignment issues if you mix and match optic mounts and magnifier mounts. Is that actually a concern for you? Maybe not. If you aren’t running a magnifier, then it certainly doesn’t make that much of a difference to you.
One more item, a lower third height still doesn’t solve using your rifle with night vision, either.
Lower Third Height Best Use Case
I think the lower third height is best for people who have the following requirements:
- You have fixed sights and want a those sights to take up less of your optic’s field of view
- Your rifle fixed rear sights
- You have bulky items on the top of your rifle and don’t want them cluttering your sight picture
- A slightly more “heads up” shooting position appeals to you
The Dark Horse: 1.93″ Height
I’m throwing this one in here simply to discuss it. I know it would come up otherwise.
On my very first AR project, I used a 1.93″ ADM scout mount for an LPVO optic. The internet told me this was the most high speed way to run my rifle because it would be so fast. This height does about half an inch over the standard. Which certainly brings your head into a more upright and natural position when standing. Your “cheek weld” becomes more like a “chin weld.”
However, about a year into things I started doing more positional shooting at things further away than 50 yards. I quickly found that as comfortable as a 1.93″ height was in the standing, it was really uncomfortable from the prone due to bending my neck upwards so much.
The 1.93″ height does not cowitness at all, it’s too high. This height is purpose-built for stand up close range gunfighting, and is the most appropriate option if you’re using night vision equipment or other protective equipment like gas masks.
Outside of those use cases, I don’t think it offers anything drastically better than the other options. The possible exception is running the Unity Tactical FAST mount, with “pop-up” magnifier. That’s just downright nifty, but is a very niche use case.
Have I answered your questions about cowitness height? Let me know if there’s anything I missed down below. An article for another day is some of the tricks you can do with cowitnessed iron sights, and why you might even consider different zeroes for your sighting systems.
In the end, whether you choose to go absolute cowitness height, lower third, or something else- it is very much a personal preference. I have used all of them, and continue to use both in different circumstances.
Great article. For a long period of time I used the folding rear BUIS with a Trijicon red dot due to astigmatism in the right eye. The round aperture will often minimize the squiggly line of the red dot to something like a red dot. Fortunately, the astigmatism has receded for now. However, BUIS’s are vitally important. Things break and get messed up in real life and off the range. I would always have BUIS on an AR, scopes are better than they used to be but guns get dropped, debris or other stuff can impact the lens, and so… Read more »
Awesome article. I trained on iron sights, so figuring out how a holographic exactly fits in with that has been confusing. Those photos are absolutely clutch. Thanks!!