Thread: Co-witnessing iron sights and red dot sight question

Originally Posted by Gunner3456

I guess I'm just stupid this morning, LOL. (Or maybe all of the time.) I really don't understand.

I can't picture how I can adjust a scope dot to the buis at the same time I'm zeroing it on a target, unless I have infinitely micro-adjustable height on my scope mount.

I still don't know how to have both sights zero on targets at my preferred distance(s) while adjusting both to each other with fixed mounts. To my weak mind that's two opposite things.

Maybe I just don't have the latest equipment, which I don't.

ALRIGHT... does ANYONE speak High-Mandarin Chinese around here?!! Let's try that!! (Just kidding!!)

Originally Posted by Stomper

ALRIGHT... does ANYONE speak High-Mandarin Chinese around here?!! Let's try that!! (Just kidding!!)

yeah, me too. ANyone care to translate a few aplhabet soup shortcuts?


I think

LOL means laugh out loud

POI is point of impact?

BUIS means...????? (maybe something something Iron SIght?)

FSP is Front Sight Post?

1.3 cowitness is ???

IBZ is translated, Improved Battle Zero, and defined. Great concept...

learning some new things here.... trying to get as much as I can. The only cure for ignorance is questions... right?


thanks for the help, guys...... appreciated. Part of why I like to hang out here... I learn a lot.

Originally Posted by tionico

yeah, me too. ANyone care to translate a few aplhabet soup shortcuts?


I think

LOL means laugh out loudYep

POI is point of impact?Yep

BUIS means...????? (maybe something something Iron SIght?)Back up iron sight

FSP is Front Sight Post?yep

1.3 cowitness is ???This one, I understand what it is, but I wouldn't know how to correctly define it

IBZ is translated, Improved Battle Zero, and defined. Great concept...

learning some new things here.... trying to get as much as I can. The only cure for ignorance is questions... right?


thanks for the help, guys...... appreciated. Part of why I like to hang out here... I learn a lot.

Here's how I do it, which may clarify or confuse.

I zero my Irons (Troy BUIS rear, Magpul MBUS front) the old fashioned way at 50 yds. Then, I slap on my optic (Aimpoint Micro H-1 on a Larue Tall mount) and, while keeping both irons up, I put my dot right on the front post of the iron. This gets me EXTREMELY close to POA/POI on the red dot, like one click up or down for absolute zero. I set it and forget it. As mentioned earlier, within 2 inches at all available distance I will ever take a shot at.

Originally Posted by tionico

yeah, me too. ANyone care to translate a few aplhabet soup shortcuts?

1.3 cowitness is ???

a 1/3 co-witness is when you look through your optic to use the red-dot (or crosshairs that is already BSZ/IBZ zeroed to a set POI) the FSP (already zeroed BSZ/IBZ to a set POI) is in the lower 1/3 of the scopes view when you use your BUIS system. 1/3 or absolute (red-dot or crosshairs sitting on the very top of the FSP) co-witness is achieved by the height of the riser/mount you use on your rails. Oh yeah, and "MBUS" means Magpul Back Up Sight.... clear as mud now?

Originally Posted by Stomper

a 1/3 co-witness is when you look through your optic to use the red-dot (or crosshairs that is already BSZ/IBZ zeroed to a set POI) the FSP (already zeroed BSZ/IBZ to a set POI) is in the lower 1/3 of the scopes view when you use your BUIS system. 1/3 or absolute (red-dot or crosshairs sitting on the very top of the FSP) co-witness is achieved by the height of the riser/mount you use on your rails. Oh yeah, and "MBUS" means Magpul Back Up Sight.... clear as mud now?

Well I've been googling the heck out of this and what I still believe is that the height above the barrel of both the optic and the iron sights must be the same to get the fsp and red dot to agree when aiming with either or both the BUIS and the optic. I repeat my analogy of putting a typical scope on a 30-30 and trying to even see the irons through the optic.

It does appear to me to be true that the BUIS on an AR are taller than on a 30-30. It might also be true that the optic mounts on the rail on the AR are shorter/lower than on some other rifles but I didn't measure any.

I will say that I couldn't find a definitive answer to this on google.

I have no rifle other than my AR's which will allow me to see the BUIS through the optic.

I think you could reverse the statement, and say that if the BUIS and optic are both the same height above the barrel, and you sight them independently at the same target, you will see the fsp aligned with the red dot when shooting.

I never really gave this detail any thought before.

Originally Posted by Gunner3456

Well I've been googling the heck out of this and what I still believe is that the height above the barrel of both the optic and the iron sights must be the same to get the fsp and red dot to agree when aiming with either or both the BUIS and the optic. I repeat my analogy of putting a typical scope on a 30-30 and trying to even see the irons through the optic.

It does appear to me to be true that the BUIS on an AR are taller than on a 30-30. It might also be true that the optic mounts on the rail on the AR are shorter/lower than on some other rifles but I didn't measure any.

I will say that I couldn't find a definitive answer to this on google.

I have no rifle other than my AR's which will allow me to see the BUIS through the optic.

I think you could reverse the statement, and say that if the BUIS and optic are both the same height above the barrel, and you sight them independently at the same target, you will see the fsp aligned with the red dot when shooting.

I never really gave this detail any thought before.

I realized last night that the distance from the CL of the barrel to the CL of the optic doesn't actually matter,
what matters is the CL of the barrel to the RETICLE.

If you can see your irons through the scope, the reticle will be collinear with your irons when they are sighted in for the same distance, so they will coincide - unless you have to move your head to see the reticle. You shouldn't if you can view the irons through the glass.

On the 30-30, they will still coincide at the sight in distance, but the distance between the line of the iron sights and the reticle will not coincide elsewhere - the sight in distance/target will be the point of convergence.

Originally Posted by toolfan

I realized last night that the distance from the CL of the barrel to the CL of the optic doesn't actually matter,
what matters is the CL of the barrel to the RETICLE.

If you can see your irons through the scope, the reticle will be collinear with your irons when they are sighted in for the same distance, so they will coincide - unless you have to move your head to see the reticle. You shouldn't if you can view the irons through the glass.

On the 30-30, they will still coincide at the sight in distance, but the distance between the line of the iron sights and the reticle will not coincide elsewhere - the sight in distance/target will be the point of convergence.

I'm still lost. I have a 30-30 with a scope, and of course the scope is a side mount due to top eject. The advantage is that I can see either through the scope or the irons. Even so the scope is right above the sights.

There is no way in heck that you can see those iron sights through the scope regardless of the aim point of either. The scope is simply too high above the irons.

I admit I probably don't know what I'm talking about and that we are probably speaking in different languages, but the answer to my dilemma hasn't been explained here yet.

That rifle has both the irons and the scope sighted in on zero at 100 yards. I can think of no, no, no way you're ever going to see the irons through the scope. I believe I'd have to replace the irons with some which are much taller - the same height above the bore center line as the scope.

OK, keep it coming...

the conversation is really only applicable to AR's not to 30-30s and similar, where there is ,for the most part, a standard in BUIS sight height,in tandem with the use of a non magnified, non prismatic optic such an aimpoint or eotech and both are sitting on a rail or rails on the same plane.

Generally speaking, if your irons are set to POA=POI at 50yrds, and whilst looking through both irons, you adjust the red dot so as it sits at your POA on the FSP, then the red dot will also be zeroed at 50yrds (within about 2" if it's a 4 MOA dot).

I prefer to zero both at 50 yrds independantly however, just because....
As mentioned before, the height of your co witness is a function of the height of the mount give or take any risers.

I have a FAL, the irons are set very low relative to the mounting point of my optic to the extent that most of my mounts block the view through them and co witness is impossible.

All my AR's have folding front sights so co-witness is not something I care about. So long as my optic and irons are zeroed correctly I'm happy

Originally Posted by asiparks

the conversation is really only applicable to AR's not to 30-30s and similar, where there is ,for the most part, a standard in BUIS sight height,in tandem with the use of a non magnified, non prismatic optic such an aimpoint or eotech.

Generally speaking, if your irons are set to POA=POI at 50yrds, and whilst looking through both irons, you adjust the red dot so as it sits at your POA on the FSP, then the red dot will also be zeroed at 50yrds (within about 2" if it's a 4 MOA dot).

I prefer to zero both at 50 yrds independantly however, just because....
As mentioned before, the height of your co witness is a function of the height of the mount give or take any risers.

I have a FAL, the irons are set very low relative to the mounting point of my optic to the extent that most of my mounts block the view through them and co witness is impossible.

All my AR's have folding front sights so co-witness is not something I care about. So long as my optic and irons are zeroed correctly I'm happy

Now we're making some sense to me. Thanks.

I run Eotechs with a riser or model 516 which does not require a riser to cowitness in the lower 1/3. I sight the irons first at 50 yrds, then the red dot at 50 yrds. Both sighting systems are independent of each other. Some like actual cowitness some like the lower 1/3. You just need to find out which one works best for your eyes.