In the world of modern sporting rifles, the integration of iron sights with optics was essential for a little while and is still something that many of us want to do.
Co-witnessing, as it is commonly known, allows shooters to simultaneously utilize their primary optic while keeping their iron sights functional.
The main reason for this is redundancy – if your primary optic malfunctions, you still want to be able to aim. By aligning the sight picture of the red dot or holographic optic with the iron sights, shooters can (theoretically) seamlessly transition between sighting systems.
In this article, we will explore the advantages and considerations of co-witnessing your optic with BUIS.
What Exactly Is the Absolute Co-Witness?
An Absolute Co-Witness on an AR-15 is when your optic is centered at exactly 2.6” Height Over Bore (HOB), which is measured from the center of the barrel. 2.6” is the standard height of iron sights on mil-spec AR-15 variants and is a fairly universal standard. I’m not aware of a single AR-15 variant that comes with iron sights not at 2.6” HOB.
The idea of “absolute” co-witness is that the reticle on your optic lines up exactly with your back up iron sights (BUIS) so that if your optic malfunctions or is otherwise unusable for some reason, you can still aim with your iron sights without changing anything about your shooting position.
What Should the Absolute Co-Witness Height Be?
The height that the optic needs to sit at is 2.6 inches above the center of the barrel, or the bore. For the most part, though, when you’re shopping for mounting plates and risers they won’t specify the HOB that it will position the red dot at, simply that it is set for absolute co-witness or ⅓ co-witness.
This is one of the things that’s convenient about absolute co-witness – pretty much every manufacturer conforms to the same standard.
The height of the absolute co-witness is designed to be relatively comfortable for most people, and if you stick with the same height, you can stick with the same cheek weld, which means you can practice shooting the exact same way, every single time. The rifle can feel the same against your cheek whether you’re standing, sitting, or prone.
Red Dots Only
Co-witness is only a consideration if the optic you are using is either a reflex sight or holographic sight. In other words, only sights that truly bend no light will function properly with a co-witness. This is true even if you have an LPVO that goes down to “true” 1x. At 1x, the scope will not be able to keep the front sight in focus, so it will just be a big blur obscuring your image.
So if you’re mounting any optic with magnification, including a fixed magnification scope, co-witness is out of the question.
To Co or Not to Co
We’ll dive deeper into comparing the different types of co-witness a little further down in the article, but I wanted to quickly provide my personal perspective on co-witnessing. The primary argument for co-witnessing, whether absolute or lower third, is to maintain a certain level of functionality even if your primary optic fails.
The idea is that you should always have a back-up plan. And sure, that makes sense, but here are some things to think about:
Co-witnessing of any kind makes it so you cannot use any optic with magnification, and won’t work well with night vision, so you can greatly limit the versatility of your rifle by insisting on co-witness.
How High-Risk Is Your Red Dot?
Unless you buy an ultra cheapo red dot, then the chances of it simply malfunctioning and failing to operate right when you need it most are slim to none.
We live in a wonderful time when electronic optics are generally pretty damn reliable, and in the meantime I’ve found that having BUIS cluttering up my sight picture significantly slows down my acquisition time, meaning I’m sacrificing the utility of my primary optic for the sake of having a backup.
The other main concern is that the red dot might get damaged or broken. In this case, though, then your sight picture is pretty screwed up, is it not? You would need to remove the broken red dot before you can even see the target area properly anyway.
If You’re Going to Do It, Do It Right
A lot of what I’ve pointed out here can be mitigated by doing a couple things: first, if you’re working on an AR, invest in flip-up irons, both front and rear.
If you can just just push them down out of the way unless you need them, then suddenly you’ve got a lot more options and you aren’t sacrificing your sight picture while using your primary optic.
Second, get an optic with a QD (quick-detach) mount. That way, if your glass is broken or fogged up or has some other kind of issue, you can just quickly remove it and get on with aiming your irons.
Wrapping Up on My Soapbox
The long and short of it is this: I wouldn’t choose a red dot solely because you want to be able to co-witness with BUIS. I don’t think the tradeoff makes sense anymore.
However, if you already know you want a red dot on your AR for other reasons, or if we’re talking about a pistol, then it’s probably worth the effort and expense to get what you need (e.g. flip-up sights, QD mount, etc.) so you can co-witness.
If you want to add a night vision device or a magnifier, there’s a good chance you’ll run into space issues on your rail, since both the NVD/magnifier and the rear iron sight need to be positioned in specific places in order to function as intended.
Which Is Better: Absolute or Lower ⅓ Co-Witness Height?
Spoiler alert: neither is better. Both are good for different reasons and bad for different reasons. Here’s some more info:
Absolute co-witness is good for when you want to be able to seamlessly transition from using your primary optic to using your redundancy without having to adjust your cheek weld or other aspects of your shooting position.
The idea behind absolute co-witness is that losing functionality of your primary aiming mechanism won’t result in a tactical disadvantage of even a few seconds.
Full disclosure: I’ve never personally been in a situation where the glass on my optic was cracked and I had to keep shooting to save my life or the lives of the people around me. I hope I never am.
That said, I’ve had the privilege of running drills supervised by various law enforcement agencies, and even one by a veteran drill sergeant.
My experience with those was that there are a lot of variables that are going to affect my follow-up shots a heckuva lot more than whether I’ve got an absolute or lower third co-witness.
I give it as my personal opinion that getting pistol sights that will do an absolute co-witness with your red dot doesn’t make as much sense as doing a lower ⅓ co-witness.
It occludes a lot of your target area, interferes a lot more with your target acquisition than the front post on a rifle, and doesn’t give you really any advantages over a lower ⅓ if your red dot malfunctions.
If your shooting stance is so dialed in that you notice the difference in height and it causes you trouble when going from the red dot to the irons, then by all means, go with absolute, but otherwise I feel like it’s not worth the trade-off.
Lower ⅓ Co-Witness
Lower third co-witness brings the optic higher up so that the iron sights (which do not change height, by the way), line up in the lower portion of the red dot’s glass rather than in the center.
This clears up your sight picture a bit when you’re using the red dot, and still gives you a redundancy in case of equipment failure.
If, for whatever reason, getting a flip-up front sight is not an option for you, then I would typically recommend lower ⅓ co-witness over absolute.
That goes back to my personal experience feeling like my sight picture was unnecessarily cluttered and instinctively trying to line up my red dot with my front post before I shot.
As I mentioned above, lower ⅓ co-witness would be my basic recommendation for pistols unless there is a clear reason to do otherwise.
A lot of pistol owners don’t bother with co-witness after they put their red dot on, which 99 times out of 100 will be just fine, but all things being equal, I would rather have lower ⅓ co-witness than no co-witness.
Pros and Cons of Absolute Co-Witness Height
- Don’t have to adjust cheek weld when going from red dot to BUIS on rifle
- Standardized height that all manufacturers conform to
- Can be used to track whether your red dot is slowly losing its zero
- Can clutter up your sight picture
- Can make target acquisition take longer when you try to unnecessarily line up your irons with the red dot before taking your shot
If you wish to know how to sight a scope, then go through our YouTube tutorials.
Absolute co-witness and full co-witness are the same thing, and they both refer to the same standard height of 2.6 inches HOB. The only other option that you’ll usually hear is lower third, which refers to a set-up where the iron sights line up in the lower portion of the glass of the optic.
Lower-third co-witness height is not standard, and you’ll get different results with different manufacturers, and even more variations when you mix & match optics with mounts. Absolute co-witness is standard at 2.6 inches HOB.
Standard AR iron sights set the aimpoint at exactly 2.6 inches height-over-bore. If you want to co-witness your red dot with iron sights, you’ll need to conform to that standard in one way or another.
Co-witness is simple enough to understand: you have two options, absolute or lower third. Absolute will position your red dot so that the actual dot lines up with the iron sights when the irons are properly aligned.
Lower third will move your red dot a little higher so that the irons line up int he lower third of the red dot’s sight picture.
Whether you do absolute co-witness, lower third co-witness, or co-witness at all will depend completely on the purpose of the rifle you’re building and how you expect to use it.
Sometimes it makes a lot of sense, sometimes it doesn’t make any sense at all.