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  #1  
Old 12-27-2013, 11:58 AM
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Default Co-witnessing heights, please help

I am trying to find a good red dot to co-witness with my BUIS. My Iron sights sight at 1.375" above my picatinny rail on my AR-15. None of the red dots I am looking at list a height for their dot. It seems like a Burris Fastfire iii with a riser would work, but I am unsure of the correct riser height. I am also interested in an eotech xps2, but it seems too tall. Any thoughts?
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Old 12-27-2013, 12:09 PM
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What kind of co-witness are you looking for? 1/3? Absolute?

I'm not entirely sure about the EOTechs but they might be absolute meaning the irons are pretty much in center of the viewable area of the eotech? I don't own one so I don't know. But I do know yhm makes a riser for it so that is what I'm basing my assumptions on.

I'm sure some who has your setup will chime in soon.

Edit: From a quick Google search seems the xps2 is absolute cowitness and the exps2 (has the lever mount?) Is lower 1/3.

Last edited by 3lsmc7; 12-27-2013 at 12:14 PM..
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Old 12-27-2013, 12:21 PM
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I am looking for absolute co-witness. Eotech's claims (I believe) are based on A2 Iron sights which sit at something like 2.5"
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Old 12-27-2013, 12:35 PM
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Everyone's claims are based on standard AR sights. What are you using as BUIS?
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Old 12-27-2013, 1:04 PM
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I am using Magpul BUIS Gen 2.
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Old 12-27-2013, 1:15 PM
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Ok, so that is standard AR height. Any optic/mount that advertises absolute or lower 1/3 will be just that.
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Old 12-27-2013, 1:27 PM
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Is that the Magpul Pro mode? They seem considerably taller than Gen2 model that I have.
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Old 12-27-2013, 2:23 PM
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An AR sight is an AR sight is an AR sight, unless you have something like Troy Micros, or HK diopter sights they are all the same height.
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  #9  
Old 12-27-2013, 2:26 PM
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Quote:
Originally Posted by pistoler0 View Post
I am looking for absolute co-witness. Eotech's claims (I believe) are based on A2 Iron sights which sit at something like 2.5"
Then you are looking for the XPS models (not the EXPS models). Some of the older EOTechs are also absolute (512, 552).
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Old 12-27-2013, 2:32 PM
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Well then, I appreciate you clearing up my misconceptions.
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Old 12-27-2013, 3:03 PM
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Eotech doesn't say in their ads witch of their sights offer lower 1/3 co witness versus absolute co witness, but as said the 512 and the XPS 2 models are setup for absolute. The newer models such as the EXPS 2 and 3 that have the 7mm raised base are setup for lower 1/3 co witness. Another consideration about Eotech sights to take into account is the type of battery they use, the newer sights that have the 123 batteries may prove difficult to find batteries for as not all places carry those batteries. In a life /death scenario that is trouble. That's one reason why I like my Eotech 512, it has an absolute co witness and uses simple AA batteries I can find anywhere..
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Old 12-28-2013, 8:10 AM
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Question

I intend to set up co-witness with a Sparc. My BUIS are Troy Micro Battle flip-up.
Once I set up 1/3 properly I will drop the irons and shoot solely with the Sparc. If I were a soldier and in combat, I'd probably want 1/3, also, especially if front sight were fixed.

In my case, would it be better and easier to zero RD or irons first? Assume no zero has been set for either.

I would appreciate opinions. I was thinking of first placing red dot over front sight post to start after setting it flush with the housing.

But I think this may throw off-confused. It seems that once red dot is zeroed, if done first, that placing it over front post might only be accurate if absolute co-witness.

I am thinking out loud and not too clearly. In any event, the conceptual difference seems to be related solely to riser height and if one changes cheek weld to look through irons through RD it should not matter if 1/3 riser height or absolute co-witness. Am I correct?

Last edited by ifilef; 01-01-2014 at 3:54 PM..
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Old 12-28-2013, 8:14 AM
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Quote:
Originally Posted by pistoler0 View Post
I am trying to find a good red dot to co-witness with my BUIS. My Iron sights sight at 1.375" above my picatinny rail on my AR-15. None of the red dots I am looking at list a height for their dot. It seems like a Burris Fastfire iii with a riser would work, but I am unsure of the correct riser height. I am also interested in an eotech xps2, but it seems too tall. Any thoughts?
I just purchased a Vortex Sparc from Amazon and it includes a number of risers and a shim which may work, but I don't know enough about the topic area to assure you if it will, given your specific references. From my limited understanding, the model I purchased was designed for flat rail AR-15 so you should be able to go absolute or 1/3 co-witness. You might want to look into that RD as an alternative and it is substantially less expensive than others.

Last edited by ifilef; 12-28-2013 at 8:25 AM..
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Old 12-28-2013, 8:22 AM
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Ifilef-

I would turn the reddot off. Sight in your irons while looking through the glass of the red dot. That way your buis are sighted in. Then turn reddot on, place dot on fsp as a get you close sight in. Then sight in the reddot as if you were using reddot only and not looking through rear buis. I'm sure there are other ways, but that is what I did and has been working for me.
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Old 12-28-2013, 8:36 AM
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Siberia n's method would work although I zero both independtly.
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Old 12-28-2013, 9:03 AM
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Thanks for input thus far.

I figure if one is properly zeroed the other should follow easily. I don't want to zero both independently and probably waste time and ammo.

In making that determination, what are opinions of which zeros more easily? Assume for the question that neither have been truly zeroed, but that a default 'battle sight zero' (not sure if correct designation) has been done to the irons so that rear sight middle of horizontal plane, and front sight post is flush with housing, and that RD is fresh out of box?

Where to go from there? How would you do it? Opinions welcome.

Last edited by ifilef; 12-28-2013 at 9:28 AM..
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  #17  
Old 12-28-2013, 9:20 AM
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Zero your irons, install the red dot. zero the RDS off your irons, fire a few rounds to confirm, done.
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Old 12-28-2013, 9:31 AM
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Quote:
Originally Posted by peacedivision View Post
Zero your irons, install the red dot. zero the RDS off your irons, fire a few rounds to confirm, done.
I am going to follow your advice-for the most part.

However, wouldn't it be better to mount the RDS for 1/3 co-witness, have unit turned off, sight the irons through the RD, then turn on RD and zero it off the irons?

But wait...I guess that I would have to change cheek weld so that dot comes down to lower 1/3 and sits atop the FS post?

Sorry, I am new to this and it's a bit confusing to me.

Last edited by ifilef; 12-28-2013 at 9:51 AM..
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  #19  
Old 12-28-2013, 9:45 AM
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You're over thinking it Zero the irons, zero the RDS off the irons, shoot to confirm and call it a day.
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Old 12-28-2013, 10:04 AM
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Question

I already have RDS mounted, but with high riser for absolute co-witness...it looks pretty good without even sighting in.

I am going to have to remove from rail anyway because change of risers to enable 1/3 co-witness.

So, you are saying, that I should keep to your original advice and sight in irons first as if no RDS existed, then mount RDS and zero it to irons? I am not at the range but at home and can't wait to mount RDS anyway set for 1/3 and see straight-through vs. RD placement when using irons with RD on...

My guess is that when I change risers RD will be in middle of RD field but irons should be in lower 1/3, but then when I change my viewing angle/cheek weld for sighting picture through irons that it should be atop FPS?
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Old 12-28-2013, 10:06 AM
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Quote:
Originally Posted by ifilef View Post
I am going to follow your advice-for the most part.

However, wouldn't it be better to mount the RDS for 1/3 co-witness, have unit turned off, sight the irons through the RD, then turn on RD and zero it off the irons?

But wait...I guess that I would have to change cheek weld so that dot comes down to lower 1/3 and sits atop the FS post?

Sorry, I am new to this and it's a bit confusing to me.
You're over complicating it. Thinking too hard about it. Follow peacedivision's advice and you'll be good to go. You'll understand when you have the optic in hand.
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Old 12-28-2013, 10:08 AM
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Quote:
Originally Posted by ifilef View Post
My guess is that when I change risers RD will be in middle of RD field but irons should be in lower 1/3, but then when I change my viewing angle/cheek weld for sighting picture through irons that it should be atop FPS?

Correct
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Old 12-28-2013, 10:10 AM
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Quote:
Originally Posted by ifilef View Post
I already have RDS mounted, but with high riser for absolute co-witness...it looks pretty good without even sighting in.

I am going to have to remove from rail anyway because change of risers to enable 1/3 co-witness.

So, you are saying, that I should keep to your original advice and sight in irons first as if no RDS existed, then mount RDS and zero it to irons? I am not at the range but at home and can't wait to mount RDS anyway set for 1/3 and see straight-through vs. RD placement when using irons with RD on...

My guess is that when I change risers RD will be in middle of RD field but irons should be in lower 1/3, but then when I change my viewing angle/cheek weld for sighting picture through irons that it should be atop FPS?

Yes, you'll have to adjust your cheek weld SLIGHTLY. You're first question, that is correct.... do just that.

You don't need to be at the range to zero the RDS. The range is only needed to confirm.

Second question, yes that is correct.
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Old 12-28-2013, 10:11 AM
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Pretty simple summary
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Old 12-28-2013, 10:15 AM
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Thanks a lot for responses.

Okay, I am going to add the supplied shim to the high riser and check things out.

The problem is that neither RDS nor irons are sighted in for zero, though the irons are preset by me to a 'battle sight zero'.
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Old 12-28-2013, 10:20 AM
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Quote:
Originally Posted by peacedivision View Post


Pretty simple summary
Fine illustration, peacedivision. Thank you.
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Old 12-28-2013, 10:24 AM
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Default re: 1/3 co-witness

So, when adjusting the RD to the irons, I will have to drop line of sight so that red dot sits atop FPS in that sight picture?

Later: After inserting the shim between the high riser and the sight, and then mounting on the rail, when I look through the RDS field the RD is in center and well above FPS, and when I lower my cheek/chin slightly it does sit nearly atop the FPS. I think that I understand now after viewing a bunch of YouTube videos and this thread as that is how it should be.

I will remember the F-O-R-S acronym (front opposite, rear same) for the iron sights, but it may be opposite when mating the RD adjustment to it That will be trial and error today at the range, but I will be able to see adjustment to the front post sight reference without firing a shot. Then shoot and fine-tune.

Will also have to remember that when re-positioning red dot to match atop FPS, I must lower cheek well/line of sight to get proper iron sight picture for that adjustment. Please correct me if I am wrong.

Thanks!

Last edited by ifilef; 12-28-2013 at 11:21 AM..
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Old 12-31-2013, 7:28 AM
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Default Co-witnessing heights, please help

http://youtu.be/ogX-cIx9hpk

Hope this helps.
Might want to look at our micro dots they are great.
https://www.primaryarms.com/Red-Dot-...Arms-s/903.htm


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  #29  
Old 01-01-2014, 4:14 PM
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Yes, thank you. Have seen the video a number of times and found it to be helpful. Already have a newly-purchased Vortex Sparc.

In any event, it seems I've wasted a bit of ammo trying to zero my iron sights, which are Troy micro battlefield flip-up.

Got the rifle in a trade and noticed the rear sight DOA was set off quite a bit to the left side. I should've left it there- instead I set for a battle-sight zero and ending up shooting to the right of target at 50 yds. So it eventually ended up to the left and quite a bit.

I'm not even sure these are the correct iron sights for my JD upper with Centurion Quad Rail. I really like the rifle but I've read elsewhere micros are for higher than normal flat top rails than usual on M4. Don't really know if the quad rail fits in such a category-maybe or seems that it does not and is of standard height.

I also had initially set micro FSP (v-post, M4) for battle-sight zero, too. Another mistake.

Had to end up adjusting that numerous times as well-it is now a couple of revolutions below flush with the housing. I still am not convinced the sights are zeroed though I did hit the bullseye a couple of times in a very small Caldwell target at 50 yds.

I place the front sight post (FSP) at 6 o'clock position for purposes of sight picture.

Last edited by ifilef; 01-01-2014 at 4:19 PM..
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