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Old 06-01-2013, 01:36 PM
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Default Absolute co-witness vs. lower third co-witness.

I have purchased a Vortex Strikefire red dot sight, but I accidentally bought the version with the low mount. It is not suitable for mounting on my M4 flat top unless I remove the rear sight (which I will not do). So I need to order one of Vortex's high mounts so that I can use both the optic and the iron sights as I choose. My options are lower third co-witness or absolute co-witness, which I think means that the iron sights and the illuminated dot appear at the same level of vision.
I am wondering about the relative merits and demerits of these two options. Your input would be appreciated.
Thanks!
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Old 06-01-2013, 01:45 PM
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Lee in Quartzsite Lee in Quartzsite is offline
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I use an EOTech XPS-2 in conjunction with my Magpul BUIS. The red dot sits exactly on top of the front sight post when both are sighted properly. I like it this way because it is an instant check to see if one of the sights has somehow gotten off of its Zero. I normally shoot the gun with the iron sights folded down.
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Old 06-01-2013, 01:52 PM
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Absolute co-witness means that the iron sights will be lined up in the center of the optic. The downside is that the sight picture can be cluttered with this arrangement.

With lower third, the sight picture is not as cluttered, but the irons are still useable in case the optic fails.
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Old 06-01-2013, 02:13 PM
WR Moore WR Moore is offline
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The first reply is correct, with (what you call absolute) co-witness, the red dot should be sitting on top of, or half the dot above the front post. As noted, this provides instant verification that the sights haven't moved. Unless, the stuff has really hit the fan and both moved, which is unlikely. It's a whole lot easier than you think for gear to move the A2 rear sight-at least for windage.

The other advantage to this is that if your cheek weld is a little off, you're further above the irons and have a less cluttered view through the optic. If for some reason, your dot disappears, a slight adjustment to the cheek weld brings the irons back into alignment.
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Old 06-01-2013, 03:29 PM
gdnagle gdnagle is offline
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Not to change your subject but if you have to buy another riser how about return for the Sparc it comes with Risers and Shims you can combine in various ways to mount the SPARC at four different heights.

It is smaller close to the size of the aimpoint T1 and with the purchase of adittional riser to get you want to be might be the same price. I have the Sparc and it's about indestructable.

Just a thought.
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Old 06-01-2013, 03:50 PM
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I should have mentioned that my front sight is fixed and does not fold down.
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Old 06-01-2013, 05:24 PM
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In either case, absolute or lower third, the red dot will be on the top of the front post. The only difference is where the iron sights are within the red dot optic, either the lower third or the center.

I can see good arguing points for both ways:

Lower third places the irons out of the picture more and lets your dot float above the sights for a bigger overall picture.

Absolute has the irons centered in the optic for a fast and centered picture. The dot, the front sight and the rear sight are all centered in the optic.

.
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Old 06-01-2013, 06:29 PM
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Correct me if I am wrong, but it seems that the 'lower third' version places the red dot sight higher up than does the absolute co-witness version.

At this point I am wondering what is the need of having the red dot coincide with the sight picture. I guess that if both sights fold down it would be useful to get them all aligned, then either use the iron sights with the dot turned off or fold the sights down and use the dot. But with a fixed front sight, the red dot would always be on top of the front sight blade. The front blade would be an unwanted complication if I wanted to use the dot.

Last edited by andyo5; 06-01-2013 at 06:36 PM.
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Old 06-01-2013, 07:44 PM
crofoot629 crofoot629 is offline
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I prefer the lower 1/3.

Your cheek weld is what changes to put the lower 1/3 co-witness red dot on top of your front sight post.

I prefer fold down back up sights and depending on the situation leave them up or down. The battery life is so long on many of the Aimpoint dots, I leave my house gun with the dot on, and sights down. On a rifle put away for longer storage I turn the dot off and leave the sights up.

Back to your question I prefer the lower 1/3 as the irons are more out of the way but there if you need them.

Emory
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Last edited by crofoot629; 06-01-2013 at 07:46 PM.
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Old 06-01-2013, 11:49 PM
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Do not rely on the red dot resting on the front sight post either...They should be sighted individually. Depending on your sights, and the range you zero each for, the red dot may or may not sit on top of the front sight.
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Old 06-02-2013, 12:05 AM
crofoot629 crofoot629 is offline
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With the "lower 1/3 co-witness" your dot and irons are in fact also "absolutely witnessed" it's just your cheeck weld is different if your using the lower 1/3 so as the irons appear lower than your dot and you have a less cluttered sight picture. There no paralex offset with a dot. No matter where your head is, if you can see the dot and put the dot on the target that is where your bullet goes.

With the lower 1/3 set up you just naturally gravitate to a cheek weld that puts the irons in the lower 1/3. Then if you want, or need to, you can adjust your cheeck weld to use the irons and your dot will appear right on top of your front post.

The Voodo is in the dot being paralex free.
For this reason I see the lower 1/3 as the way to go.

And FYI I zero my AR's with Aimpoint dots at 50 yards, and then check to see that they are pretty much then dead nuts at 200 also. Works for me with 14.5" barrel and M-193.

Right or wrong, I first zero my irons and then zero my dot. It's just the way I do it.

Emory
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Last edited by crofoot629; 06-02-2013 at 12:10 AM.
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Old 06-02-2013, 10:33 AM
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Well put Emory, and that is the way I sight in and use my sights also.
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Old 06-02-2013, 10:49 AM
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Emory, I intend to get the Vortex mount that places the optic at the correct height for 'lower third' viewing of the irons, and places it 1" forward of the rail mounting point to give extra rear clearance so you can use the rear peep sight more easily. Also easier to see the 2x multiplier. Here is the URL: Vortex Optics - Cantilever 30 mm Ring Lower 1/3 Co-Witness
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Old 06-02-2013, 11:28 AM
crofoot629 crofoot629 is offline
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A couple of other things to consider.

Google "improved battle sight zero"

I like my optics mounted entirely on the rifle upper. Not on the handguard rails.

Emory
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Old 06-02-2013, 11:51 AM
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Quote:
Originally Posted by crofoot629 View Post
A couple of other things to consider.

Google "improved battle sight zero"

I like my optics mounted entirely on the rifle upper. Not on the handguard rails.

Emory
That's what I am planning on. Everything on the upper's Pic rail, not on the handguard.
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