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

Finally have my AR carbine about where I want it with YHM BUIS front and rear set and an Aimpoint CompM3. This is probably a stupid question, but that's never stopped me before.

When co-witnessing Iron sights and a red dot sight, do you dial in your iron sights first and then align your red dot or the other way around? I suppose you could do either, but it would appear to make more sense to dial in your irons first.

Any opinions on either method? Other options?

I have a different philosophy. I hate to make stupid suggestions but it's never stopped me before.

If I have a long range optic, I sight my BUIS at 100 yards as always, and then sight my optic at 2 or 300 yards. Different tools for different purposes and with different abilities.

If I have a low power optic I don't care which I sight first. I just zero both of them at 100 yards.

Originally Posted by Gunner3456

I have a different philosophy. I hate to make stupid suggestions but it's never stopped me before.

If I have a long range optic, I sight my BUIS at 100 yards as always, and then sight my optic at 2 or 300 yards. Different tools for different purposes and with different abilities.

If I have a low power optic I don't care which I sight first. I just zero both of them at 100 yards.

Your suggestion for a long range optic and BUIS makes a lot of sense or even if I had a magnifier for the CompM3.

Both zeroed at 100 yards with no magnification makes sense. I just wondered if it really made any difference which one you did first or why you might want to sight one in before the other.

If you're going to try to sight one off the other, Zero the Aimpoint first. Then adjust the irons to it.

This way, the dot will represent where you want to put the target. For instance, it can be floating above the front sight post (So you would aim just under the target with irons), right on the face of the post (Iron blocking target), etc. The red dot can represent where you want to use your irons.

If you're lower 1/3rd cowitness, just zero them individually and shoot shoot shoot!

Originally Posted by wakejoe

If you're going to try to sight one off the other, Zero the Aimpoint first. Then adjust the irons to it.

This way, the dot will represent where you want to put the target. For instance, it can be floating above the front sight post (So you would aim just under the target with irons), right on the face of the post (Iron blocking target), etc. The red dot can represent where you want to use your irons.

If you're lower 1/3rd cowitness, just zero them individually and shoot shoot shoot!

Joe, thanks! This was the answer I was looking for. I am lower 1/3 co-witness with the LaRue cantilever mount that I have the Aimpoint on. I figured you'd chime in.

I have the Magpul MBUS on my M4... I zeroed the irons to 100 meters then my x1 red-dot to sit right on top of the front sight post. Now I keep the rear sight folded and just use the red-dot, and have the option of the rear peep if the red-dot takes a dump.

Originally Posted by wakejoe

If you're going to try to sight one off the other, Zero the Aimpoint first. Then adjust the irons to it.

This way, the dot will represent where you want to put the target. For instance, it can be floating above the front sight post (So you would aim just under the target with irons), right on the face of the post (Iron blocking target), etc. The red dot can represent where you want to use your irons.

If you're lower 1/3rd cowitness, just zero them individually and shoot shoot shoot!

I didn't understand this. I can't figure out how you adjust your irons to the Aimpoint. I'm sure I'm just misunderstanding. ??

In all cases I have to adjust my sights to the target both iron and optic, and the iron sights fall where they fall within the optic. This is true no matter what distance I want to sight each to. This is true whether I want to adjust both for the same distance or different distances.

I've never known a way to cause my iron sights to be where I want them within the optic unless I don't care how they are adjusted in relationship to a target at a specific distance. For me it's either zero both sights on their targets, or adjust the irons within the optic, but not both.

Maybe I'm living in the past without some current knowledge of equipment, but I have to zero each sight to a target at a chosen distance(s,) and the irons fall where they fall in relation to the optic.

What am I missing? No disrespect intended...

Originally Posted by SheepDog223

On my AR I have it zero'd with the IBZ method. Improved Battle Zero. Zero it to 50yds. The projectile will be on target at 50 and 290 yds and will never be more that 2" off anywhere inbetween the two points. I zero my 3x9 scope to 100 yds and use the bdc reticle from there. I zero my reflex to 50 and 290 also. Just another thought for you to consider.

As with a scope, this works for me only if the sights are a predetermined distance above the barrel. The higher the sights are above the barrel, the greater the angle of convergence. I have an AR with lower (to the barrel) sights that won't "obey" that formula. I have to sight at 100 yards to hold close to the above specs at distance. My bullet simply doesn't climb as steeply leaving the muzzle.

It also works only with a given weight and speed of bullet because different ones will have a different trajectory.

For me, I have to test the formula with actual firing to see what it takes to bring the bullet back to close to zero at 300 yards and to therefore keep it always within that roughly 2" or so zone of trajectory.

$.02

For my red-dot x1 optic on my flat top M4 I actually had to buy a 1/2" riser to put under it to get it to co-witness with my FSP otherwise it was too low on the FSP, plus too low to even look through. After I had the 1/2" riser between the optic and receiver rails it wound up being dead center of the optic glass, then it was just a matter of visually adjusting the red-dot with the windage elevation knobs to sit right on top of the FSP.

Originally Posted by Stomper

For my red-dot x1 optic on my flat top M4 I actually had to buy a 1/2" riser to put under it to get it to co-witness with my FSP otherwise it was too low on the FSP, plus too low to even look through.

That matches my experience.

After I had the 1/2" riser between the optic and receiver rails it wound up being dead center of the optic glass, then it was just a matter of visually adjusting the red-dot with the windage elevation knobs to sit right on top of the FSP.

I'm still struggling to understand how I'd adjust my red-dot to sit right on top of the FSP while at the same time sighting the BUIS and optic to each zero on a target(s) at a certain distance(s.)

I can adjust my iron and optic sights to independently zero on a target(s) at the distances I want, or I can adjust them to each other, but not both.

??

Originally Posted by Cougfan2

Joe, thanks! This was the answer I was looking for. I am lower 1/3 co-witness with the LaRue cantilever mount that I have the Aimpoint on. I figured you'd chime in.

Great setup!

Try a zero at 50 yards. It's the "sweet spot" for a carbine, I have come to find.

I found that I had to zero in my BUIS, then while mounting the optic I arranged the reticle sight picture to be as visually close to dead center on the FSP, then locked into place on the rails, after that I could "move" the red-dot with the reticle via the windage/elevation knobs to sit right on top of the FSP while looking through the rear peep, thus giving me the "bullet hits where the red-dot is" function. Hope that helps!

Originally Posted by wakejoe

Great setup!

Try a zero at 50 yards. It's the "sweet spot" for a carbine, I have come to find.

Depending on the type/brand of sights/mounts you have and their distance above the barrel. The angle of convergence (steepness of climb of bullet leaving barrel) has a huge affect on this, as does bullet weight and speed. There are too many variables for one size fits all.

With absolutely no other information available as to weight and speed of bullet or height of sights above the barrel in that carbine, and with no time or ability to test the gun at various distances for trajectory, I'd rather sight it in at 300 yards and go home. At least I'd know I had a "decent" trajectory in a kill zone from muzzle out to at least 400 yards.

$.02

Originally Posted by Stomper

I found that I had to zero in my BUIS, then while mounting the optic I arranged the reticle sight picture to be as visually close to dead center on the FSP, then locked into place on the rails, after that I could "move" the red-dot with the reticle via the windage/elevation knobs to sit right on top of the FSP while looking through the rear peep, thus giving me the "bullet hits where the red-dot is" function. Hope that helps!

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.

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.

When you view your iron sights, do you look through your optic?

If so, and they are sighted in for the same distance, it follows that they will be adjusted to each other, doesn't it?

Originally Posted by toolfan

When you view your iron sights, do you look through your optic?

If so, and they are sighted in for the same distance, it follows that they will be adjusted to each other, doesn't it?

Not unless their height as a parallel line is exactly equal above the barrel.

I can see the sights through the optic. But If I set both for zero at 100 yards, the top of the front sight may not be near to the dot or crosshairs of the optic. It may be way above or below the dot.

If I sight in both the optic and the BUIS for 100 yards and the front sight is way below or above the dot in the optic, I can't change the BUIS to match the dot in the optic without throwing off the aim of the BUIS.

Originally Posted by Gunner3456

Not unless their height as a parallel line is exactly equal above the barrel.

I can see the sights through the optic. But If I set both for zero at 100 yards, the top of the front sight may not be near to the dot or crosshairs of the optic. It may be way above or below the dot.

If I sight in both the optic and the BUIS for 100 yards and the front sight is way below or above the dot in the optic, I can't change the BUIS to match the dot in the optic without throwing off the aim of the BUIS.

If your iron sites are on at 100 yds, and you look through your optic to see them, your optic reticle better fall on the same point of the target, otherwise, they aren't sighted in at the same point.

Originally Posted by toolfan

If your iron sites are on at 100 yds, and you look through your optic to see them, your optic reticle better fall on the same point of the target, otherwise, they aren't sighted in at the same point.

Put a scope on a normal rifle with sights very low on the gun. Say a 30-30. Put a scope on it and the scope will be so high you can't even see the sights through the scope. Now start adjusting one or the other until you can see the front sight in the center of the scope. It just can't be done.

Your hypothesis works only if the height of the line of sight parallel to the barrel of both the optic and the BUIS are identical in height above the barrel.

Originally Posted by Gunner3456

Put a scope on a normal rifle with sights very low on the gun. Say a 30-30. Put a scope on it and the scope will be so high you can't even see the sights through the scope. Now start adjusting one or the other until you can see the front sight in the center of the scope. It just can't be done.

Your hypothesis works only if the height of the line of sight parallel to the barrel of both the optic and the BUIS are identical in height above the barrel.

No, it always works when you can see the irons through the optic.

Think about it - if they are sighted in for the same distance/POI, they HAVE to be coincident.

At distances OTHER than the sight in distance, they will still appear coincident, but one will be closer to the POI than the other if there are differences in height above barrel CL.

If you mounted your 30/30 to a rigid vise, you'd find that the scope and irons was coincident at the sight in distance, but since they are mounted far from each other (relatively) you might find them not to agree at other distances.

Thanks to everyone that has chimed in. That's what I love about this site. You've all given me plenty of options and things to think about, not only for this rifle, but others I have as well.