Inability to co-witness

[steve100]

I sighted in my buis and my eotech red dot independently however they did not line up when I finished. Any suggestions?

Thank you for the help.

[Belmont31R]

Two sighting systems not meant to be used together. They don't have to match up but each one should put a bullet where you want it.

[JSantoro]

There's nothing written anywhere by any competent authority that states that a RDS and one's irons HAVE to line up. There's a lot of potential variations in the optics, the irons, the receiver, the rail(s), etc. that can add up and make it so that they do not line up.

If they do, sweet.

If they don't.....they don't. So long as each achieves hits at the distances at which they are zeroed, you're cool like school.

Shoot the gun, and be worry-free over what amounts to nothing more than aesthetics.

[Cameron]

I don't get it. If the RDS and the BUIS are both zeroed for the same distance wouldn't they have to line up?

I have a couple of carbines that have an EOTech and an Aimpoint Micro, both carbines are zeroed at 100yds with RDS and BUIS and the dots sit exactly on top of the front post. As both RDS are essentially parallax free it doesn't matter where in the window the dot is, so as I run mine with a lower 1/3 co-witness, in order to line the irons up the dot sits lower in the window but still sits right on the front post.

Cameron

[steve100]

Although it is not clear to me why my irons and optics do not line up as long as both are independently centered I will not stress about it. However I would appreciate it if anyone can explain this or provide a link to an explanation.

[JSantoro]

Are you using an FSB or a front sight mounted to a FF railed forearm?

In my experience, it's most commonly seen on the latter, and most often due to the forearm not being mounted completely plumb and true, which can put the dot off the front sight tip in either or both the horizontal and vertical plane. My own Noveske carbine, with Troy flips and a Comp ML3...the dot is off to the right and perhaps a click or two low; both are zeroed to 50yds, and each prints in the same basic spot @ 205m, just like they're supposed to.

Canted FSBs, optic mount rings that aren't true, grandma at the milling machine that was finishing the upper receiver was nodding off....any of these can create something that's a bit off, which can be compensated for or adjusted out, but sometimes we randomly end up with a perfect storm of tolerance-stacking between separate components that creates a noticeably visual aberration. This is kinda like the "ejection pattern" nonsense; if the gun ain't choking, how gives a rat's patoot where the brass is? It doesn't tell anybody anything until there is a malfunction present, at which point they can help diagnose cause, but not provide a complete explaination.

It's easy to get vapor-locked on perfect-world technical descriptions and pictures, and forget that reality bites and that Murphy was an optimist, is all. "Co-witness" describes separate sighting systems of no specific type on the same gun, zeroed to the same point; could be an optic and a side-rail-mounted laser.....also described as a "co-alignment," or "co-axial." There's no way to get something like to line up, as seen from behind the gun, so the point that defines it is downrange, at the shooter's desired final resting spot for the bullet.

If your rounds are going to the same spot, you're co-witnessed. Aligning a dot with a front sight post, or vice versa, is a starting point from which you refine where the bullet ends up. It's not the ultimate goal, and the idea that that IS....is the result of the story changing slightly every time its told, until it's not the same story any more.

[steve100]

Thank you for the explanation. It is a flip up front sight mounted on a railed forearm as you suggested

[TehLlama]

Thank you for the explanation. It is a flip up front sight mounted on a railed forearm as you suggested

Just to verify, it's on a FF handguard, not mounted to a drop-in handguard mounted against a low profile or low railed block? If the handguard moves, that will stop it, but these odd setups aren't common anymore.

[shootist~]

Did you sight in both at at same time, using the same shooting position? For example: the difference in shooting prone with a butt bag & bi-pod for the irons Vs from the bench with sandbags (or something along those lines); could account for some of be the issue. Adding in a difference in the rifles cant from the two positions also changes things.