BUIS Through An Optic – Check Your Zero!

Conventional wisdom for optics, despite their phenomenal track record of performance (at least on the high-end stuff) has dictated that a combat carbine should include a set of back-up iron sights. The operating theory is that if/when your main optic goes down, one can easily deploy the iron sights and resume aimed, accurate fire.

This is all well and good (and has come in handy for me at a 3-gun match when my main optic pooped itself), but few know that your main optic can spoil your BUIS sight picture. Simply put, your optic distorts light (as a function of being an optic), which can, in turn, distort your front sight when viewing through the optic. As such, if you zeroed without your optic in view, your zero can be off when viewed through the optic.

As a general rule, this is relatively minor with absolute co-witness, but those using lower 1/3 co-witness should pay special close attention. Almost all optics like the popular Aimpoint and various knock-offs used curved lenses to reflect the dot back to the shooter’s eye from the projector. When looking through the optic, this is not an issue, as the design accounts for the curved lenses to the projector.

But, the rear sight is out of position relative to the projector and when the sight is near the bottom of the curved lens, the curvature presents the most distortion of the light coming into the lens, which can move the front sight relative to its original non-optic-passthrough position, resulting in changed zero.

As such, if one intends to shoot through their optic with BUIS, make sure to zero with the optic in the plane to account for this. Or, have your optic on a Q/D mount so when it fails, it comes off the gun in a hurry.

KitBadger shows the phenomenon with a Trijicon SRO:

Nathan S

One of TFB’s resident Jarheads, Nathan now works within the firearms industry. A consecutive Marine rifle and pistol expert, he enjoys local 3-gun, NFA, gunsmithing, MSR’s, & high-speed gear. Nathan has traveled to over 30 countries working with US DoD & foreign MoDs.

The above post is my opinion and does not reflect the views of any company or organization.


  • Caffeinated

    You can explain it akin to bow fishing or viewing objects under water from a position above water. Any medium will bend light as it passes through. Now start adding inexpensive optics with all kinds of defects and aberrations in the material and it only gets worse.

  • Martin Grønsdal

    We, who operationally operate, don’t have to take this into account, neither do we have to zero our BUIS through the optic. We automatically adjust the shots for all thinkable corrections required to always produce a headshot. That is even when we shoot at tin boxes.

    • valorius

      LMAO 😀

    • Bill

      I think that the hip and trendy Cool Kids have quit running BUIS anyway, because reliability.

  • Major Tom

    This sounds like reason to start looking at inventing or investing in Heads-Up Displays (HUD).

    • Paul Epstein

      Zeroing a HUD for a detached firearm is a significant problem. If you can invent a technology that will repeatedly link the HUD up to the actual trajectory of the gun that’s both as accurate as current weapon sights and as reliable, that will be a major advance in the firearms field.

      • Major Tom

        And that’s what I’m getting at. A HUD system that would link a targeting reticule with the (relative) trajectory of where I’m aiming assuming I’m not using the sights. (It would also be reinforcement for when I am using the sights.)

        Might require some linking technologies on the part of the gun. I’m not sure the tech exists to where I could pick up a random AK off the street and it would calculate zero, trajectory and estimated accuracy and feed it into the display yet. I could see it with a sort of IFF type transponder that could detect orientation embedded into the gun.

        • Roger V. Tranfaglia

          Hasn’t Aimpoint done that? Using a “smart” phone/tablet with bluetooth/wifi connection. Last I read Aimpoint is back in business.

  • Joe

    Good article. I was wondering why my Aimpoint reticle seemed to be on the right side of my front sight post. Both shoot zero, but I figured the Aimpoint was “more zeroed” because my rear BUIS has less precise adjustment clicks. Now I know it’s probably just distortion.

  • gordon

    I use an offset to account for the possibility that the primary is damaged and either out of alignment or has disturbed/cracked lenses. I don’t know what the chance of that is but an offset set of irons also allows near instant transition – no having to flip them up.

  • TJbrena

    I feel embarrassed asking, but what exactly is lower 1/3 cowitness?

    • ARCNA442

      When the iron sights are in the bottom ⅓ of the optic’s lens as opposed to absolute cowitness where the irons are centered in the optic. Lower ⅓ is generally used when combining a red dot with fixed irons as a way to get a less cluttered sight picture.

      • Roger V. Tranfaglia

        Thank You!!

    • valorius

      The picture above shows 1/3 cowitness.

    • MrBrassporkchop

      Legal term for a hostile witness with dwarfism.

    • Bill

      Don’t feel embarrassed. I’ve always found it counterintuitive. I don’t even think lower third is technically a “co-witness.”

      • int19h

        You’re still “witnessing” both your irons and your red dot at once, aimed at the same target.

  • Alex Brown

    Sounds like a point in favor of 45 degree backups.

    • John Yossarian

      I’d prefer a return-to-zero QD mount for the primary optic. This allows you to remove the primary optic from your field of view if it fails and allows you to practice with or zero the BUIS with the primary optic removed.

    • Frank Grimes

      Or good optics.

      No issues through my Aimpoint.

  • Sam Damiano

    What is wrong with Iron Sights? Thousands of people can use them to hit a MOA inch X ring out to 1K.

    • Risky

      Those people have excellent eye sight, precision motor skills, and dozens of years of experience. I have none of those things, so I buy optics.

    • Ron

      There are three distinct points when talking sighting for combat weapons. 1) rifle range performance is not an indicator of combat performance. 2) most of the time sighted fire is not required because you are shooting at the general area of poorly defined or not seen target 3) the rare instance where you can apply sighted fire, it more than likely will be in poor light conditions, against a limited exposure, moving target.

      • Sam Damiano

        As I asked above are you using both sighting systems as above? My experience is limited to space clearing and security on ships, most of it shotgun or pistol and sights were fairly worthless in that setting. Current topside rovers are about the only watch stander who has more than a few yards of range.

        • Ron

          I have used iron sights, RCOs, MGOs (the old M145, not the ACOG based one) and eo-techs in combat during OIF and OEF deployments. I think the indexing provided by the sights are actually somewhat useful, but I never remember a hard focus on the sights like I did in training

        • James T Kirk

          So you say “what wrong with iron sights 1k moa shots” based off of your experiences pulling ship duty? What the hell man, that makes no sense.

  • Michael Lindner

    Several have mentioned 45 degree BUIS or QD mounts for optics. I second the thought. If you have zeroed your BUIS through your optics, and then the optic fails because the glass moves or breaks, you are SOL. If the BUIS is independent of the optic you can switch to it.

  • Sam Damiano

    Pre ACOG marksmanship in uniform was iron sights, not always optimal but a skill to learn. Now you have Soldiers and Marines who have to learn iron sights as a back up to the ACOG.
    Range finding optics with out a doubt have a place but do you use both sights at once as pictured?

    • James T Kirk

      I’m not really following you Sam, let me clarify my point and maybe that will help you. I will admit I do not agree with the current instruction for learning on the standard M4 with the ACOG. I feel it undercuts the fundamentals of understanding marksmanship and ballistics. I believe it has reduced the inherent accuracy of our basic infantry solider.

      I am only speaking to 1000m+ shots, such as those taken by men in DMR and sniper roles. These shots aren’t taken with M4 and they usually always are taken with more specialized weaponry. In these situations, some of these weapons do not even have back up iron sites, however, most do (I am referring to some of the bolt action rifles that are now being cycled out of issue).

      I was speaking to these chest pounders out there that are all like “back in the day iron sites blah blah blah 5 mile head shots”. There is no getting around that longer distance shots are greatly aided by optics particularly ones with range estimation built it. This ups or increases first shot hit probability immensely, while almost guaranteeing second round follow up hits. Iron sites are great, I’m NOT discounting their rugged nature or saying they do not have a use, I am simply saying that people that advocate their use on longer shots have no idea about how much additional stress they add to long distance shots IN THE FIELD.

      I believe those people you see bragging about 1000m shots with iron sites have never taken a shot in combat and are only speaking from their experience from a bench in a National Match competition.