Mounting Aimpoint: Co-Witness or Quick-Detach?


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JJNA
March 14, 2013, 08:22 PM
I am seeking advice about mounting an Aimpoint on a carbine. I plan to use the Larue quick-detach mount, but have not decided on the height.

My carbine has backup iron sights that are flip-up and low-profile when flipped down.

The question is, should I opt for a taller mount so that I can get a 1/3 co-witness with the BUIS (flipped up) OR should I go with the shorted possible mount so that the dot sits closer to the bore (and also affords me a better cheek-to-stock contact)?

The first option seems to give me simplicity -- if the red dot failed, I could simply flip up the BUIS and continue. On the other hand, I am concerned this might force me into more of a chin-to-stock contact.

The second option gets me a closer-to-bore mount. If the red dot failed, I'd use the red dot tube as a crude ghost ring sight and/or quickly detach the red dot, flip up the BUIS and continue.

I'd appreciate any advice. Thank you.

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Captains1911
March 14, 2013, 11:00 PM
Whether you chose a lower 1/3 or absolute, you still sight though the optic when using the BUIS (no need to remove it). Its more personal prefernce than anything else. The cheek weld difference between the two is hardly noticeable. Since you can flip the BUIS down out of the way when using the optic, I would chose an absolute co-witness. That will give you a better sight picture when using the BUIS because the the sights will be centered in the optic rather than in the bottom third of the view.

Absolute co-witness:
http://i38.photobucket.com/albums/e106/Captains1911/paper%20punchers/DSC00689.jpg

Lower 1/3 co-witness:
http://i38.photobucket.com/albums/e106/Captains1911/paper%20punchers/DSC01011.jpg

JJNA
March 15, 2013, 12:37 AM
Thank you for the reply, but you misunderstood me. I am not considering absolute co-witness. The choice is between 1) 1/3 co-witness on Larue QD mount or 2) lowest Larue QD mount (with which I would not be able to use the flip up BUIS unless I took the optic off first).

MrCleanOK
March 15, 2013, 01:17 AM
Remove your optic to be able to use your BUIS? No thanks. If you don't need to be able to use your BUIS immediately, you probably don't even need them in the first place.

I would rank your options, best to worst (and yes, I know my #1 isn't on your list):

1. Absolute co-witness
2. 1/3 co-witness
3. No co-witness

Captains1911
March 15, 2013, 08:55 AM
Thank you for the reply, but you misunderstood me. I am not considering absolute co-witness. The choice is between 1) 1/3 co-witness on Larue QD mount or 2) lowest Larue QD mount (with which I would not be able to use the flip up BUIS unless I took the optic off first).
I don't know what low LT mount you are referring to, all of their Aimpoint mounts are designed for either an absolute or lower 1/3 co-witness. It doesn't make sense to do what you are asking about. What specific Aimpoint and mount are you referring to?

JJNA
March 15, 2013, 04:37 PM
http://www.laruetactical.com/larue-tactical-aimpoint-micro-mount-lt660-lt660hk-or-661

LT661 for Aimpoint T1.

Captains1911
March 15, 2013, 04:39 PM
http://www.laruetactical.com/larue-tactical-aimpoint-micro-mount-lt660-lt660hk-or-661

LT661 for Aimpoint T1.
That mount is designed for use on AKs and shotguns. You should be looking at either the LT751 or LT660.

browneu
March 15, 2013, 04:40 PM
Whether you chose a lower 1/3 or absolute, you still sight though the optic when using the BUIS (no need to remove it). Its more personal prefernce than anything else. The cheek weld difference between the two is hardly noticeable. Since you can flip the BUIS down out of the way when using the optic, I would chose an absolute co-witness. That will give you a better sight picture when using the BUIS because the the sights will be centered in the optic rather than in the bottom third of the view.

Absolute co-witness:
http://i38.photobucket.com/albums/e106/Captains1911/paper%20punchers/DSC00689.jpg

Lower 1/3 co-witness:
http://i38.photobucket.com/albums/e106/Captains1911/paper%20punchers/DSC01011.jpg

Thank you for the sight pictures.

Sent from my SGH-T999 using Tapatalk 2

JJNA
March 15, 2013, 04:43 PM
I have a SIG 556 Commando with flip-up BUIS. Sorry, I should've mentioned that earlier. It's pretty much like a flat top, so I was wondering whether it might be a good idea to simply get the lowest mount possible.

hso
March 16, 2013, 07:02 AM
I was wondering whether it might be a good idea to simply get the lowest mount possible.

No

As pointed out you should set up for either an absolute cowitness or 1/3 cowitness so you have a consistent cheek weld and use of the rifle.

JJNA
March 16, 2013, 08:02 AM
No

As pointed out you should set up for either an absolute cowitness or 1/3 cowitness so you have a consistent cheek weld and use of the rifle.
What's wrong with getting the lowest mount possible? And, in a very rare chance the Aimpoint fails, detaching it (it would be Larue QD mount), flipping up the BUIS and using the latter?

MrCleanOK
March 16, 2013, 10:42 AM
Well, for one, anything lower than the sight plane of the iron sights (absolute cowitness) is probably too low to use comfortably with a good cheek weld. There is a reason that AR sights are the height they are. You can go higher, at the expense of a consistent cheek/chin weld, but it's not really practical to go lower. If you need proof, there is a picture on the product descritpion for the mount you are looking at with the answer clearly stated in red. "Will not work properly with standard AR-15/M-16 uppers". I don't think Larue could be any clearer than that.

JJNA
March 17, 2013, 04:47 AM
Well, for one, anything lower than the sight plane of the iron sights (absolute cowitness) is probably too low to use comfortably with a good cheek weld. There is a reason that AR sights are the height they are. You can go higher, at the expense of a consistent cheek/chin weld, but it's not really practical to go lower. If you need proof, there is a picture on the product descritpion for the mount you are looking at with the answer clearly stated in red. "Will not work properly with standard AR-15/M-16 uppers". I don't think Larue could be any clearer than that.
I am NOT using an AR-15. With the SIG 556, I think the low mount is the close thing to the BUIS height (a little lower perhaps) while the medium mount (HK) may give me 1/4 or 1/5 co-witness. The latter may make the optic too high for a cheek-weld (I tend to crunch down hard on the stock with my cheek).

Traditionally, has it not been the advice to put an optic as close (low) to the boreline as possible?

I understand that real gunslingers in ninja oufits -- when seconds matter in gun fights -- need to be able to shift to BUIS (or fixed sights) rapidly should their optics fail. But for me as a civilian in a decidedly less than a dangerous setting, what's wrong with mounting the optic as low to the rail as possible and then -- in a very rare possibility the red dot would fail -- quick-detach the optic and flip up the BUIS?

Captains1911
March 17, 2013, 11:00 AM
I had AR in my head. I'm not familiar at all with the Sig 556. The low mount may or may no work. From looking at pics I suspect that even with the BUIS folded down, they may still be in the way with the optic that low. Your best bet is to give LaRue and call and ask.

JJNA
March 17, 2013, 08:18 PM
I had AR in my head. I'm not familiar at all with the Sig 556. The low mount may or may no work. From looking at pics I suspect that even with the BUIS folded down, they may still be in the way with the optic that low. Your best bet is to give LaRue and call and ask.
From the measurements I obtained from the Internet it seems the low mount will be fine on SIG 556 with the BUIS flipped down. I originally posted here, though, to see if anyone had a personal experience with that setup -- you know, since measurements and how things work in reality are not always the same. Thank you for all the replies. Larue recommended the medium height (HK) mount, but I think it's because the folks there are pretty wedded to the 1/3 co-witness.

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