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9/22/2017 12:11:25 AM
Posted: 8/2/2005 5:30:22 PM EDT
[Last Edit: 8/2/2005 5:30:52 PM EDT by Chestwound]
I just got an ARMS throw mount with spacer for my Aimpoint M2 that makes it absolute cowitness. Seems fine to me however I have a short window of time where I could switch the spacer (at no add'l cost or hassle) to the lower 3rd co-witness if desired. I want to be sure to choose what's best - assuming one is actually better than the other.

I've read and understand the difference in sight picture between Absolute Co-witness and lower 3rd Co-witness. From a tactical or other standpoint is there an advantage of one of the two over the other? Is it all simply a matter of personal preference?

Thanks in advance for your help.

PS: I have both the Aimpoint and a Trijicon Accupoint TR21R. Both are on quick disconnect throw mounts so I can quickly interchange them with ease on my Bushmaster M4. At home for extreme CQB defensive purposes I prefer to keep the Aimpoint on (max shot range would be 30-40 feet indoors - hope I never have to take that shot). Outside, stored on the boat and at the range I much prefer the Trijicon. The witness question only applies to the Aimpoint since with the Trijicon I can't "see" the FSB even at the lowest 1.25X magnification setting and it does not witness either way.....

Link Posted: 8/2/2005 8:23:06 PM EDT
Link Posted: 8/2/2005 8:50:36 PM EDT
Lower third gives you both options.

Absolute co witness does not.
Link Posted: 8/3/2005 3:50:00 AM EDT

Originally Posted By Boom:
The lower 3rd mounting provides you with a better field of view. Less tower in your picture and more target.





+1

­but i think the half spacer would be too short for cowitnessing, if thats what spacer your referring to.
Link Posted: 8/3/2005 5:54:58 AM EDT
Right off the bat, you need to understand that individual humans vary immensely in how they process optical data. Some guys I've seen can deal with both fixed iron sights right in the central sight plane with no problem. It drives me nuckin futz. I got by OK with the fixed front sight and a flip-up rear iron. Some guys can't tolerate even the fixed front iron in the main line of sight.

I switched to the lower third cowitness setup and fixed front and rear irons for one reason: speed. If the red dot doesn't appear, you have no decisions to make regarding flipping up rear (+/or front) iron sights. Just scrunch down your cheek weld and get back in the war RFN!

Luck.

SD
Link Posted: 8/3/2005 6:21:16 AM EDT
Absolute co-witness allows for the same cheek weld with both the optics and the irons.
Link Posted: 8/3/2005 6:21:17 AM EDT
Oddly enough, I had this very conversation with Ken Hackathorn this last weekend about this. His point was that the dot scope is for fast acquisition and wide-open field of view of the target. His suggestion was that the BUIS should be viewable in the lower 1/3 of the dot scope and the BUIS should be used for ranging shots out further than the dot MOA is good for. I got to shoot his HK416 upper with an Aimpoint and it was set up this way and it worked pretty well.
Link Posted: 8/3/2005 9:40:15 AM EDT
Link Posted: 8/3/2005 11:31:43 AM EDT
It's personal preference. My prefernce happens to be "lower 1/3".
Link Posted: 8/3/2005 2:48:01 PM EDT

Originally Posted By C4iGrant:

It is funny that you say that your ARMS #22M68 and full spacer gives a "absolute" co-witness. There are many on this board that would disagree with you on this.
C4
www.GRTactical.com



I based that description on the pic in the BUIS FAQ here on AR15. With the smaller spacer the top of my FSB and the rear aperture look like the one described here - basically in the center of the optic.

http://groups.msn.com/isapi/fetch.dll?action=MyPhotos_GetPubPhoto&PhotoID=nJwAAANwMUuAJ7JC8BIu3u5hfwsqH9Z0x3oIwsknAVfVq­koJ0B820UgDvJdctqwwGbFtqpVZEnbk
Link Posted: 8/3/2005 2:51:55 PM EDT
I've used both with sucess. That said, I prefer the 1/3 method for its field of view and the higher height of the optic. I also prefer flip sights front and rear, for a completely clean FOV. YMMV.
Link Posted: 8/3/2005 5:49:34 PM EDT
Thanks to all who replied. The 100% concensus favorite among you is the lower 1/3rd. I understand the rationale. Since the overwhelming majority of the time I have the Aimpoint on it will be the primary sight (not the irons), I'd rather not have the FSB tower impeding so much of it's FOV. As someone else said, the BUIS are supposed to be just that when you have optics - BACKUPS. I swapped out the spacer and am now part of the lower third crowd.

Link Posted: 8/4/2005 8:47:38 AM EDT
[Last Edit: 8/4/2005 8:52:15 AM EDT by Redhook]
Regardless of the theory behind co-witness, I think there is an equipment issue here. Maybe I'm missing something but, It's my understanding that the best you can do with an ARMS 22m68 mount and a full spacer is absolute co-witness. Unless ARMS is making a taller spacer now, you can't get the aimpoint up high enough to get the irons in the lower third. I though that was the reason that everyone was switching to the tall LaRue mount.

ETA: That is unless you've mounted the 22m68 up on a riser like a SIR rail w/o a spacer then added the 1/2 spacer. I guess that would get you up the extra distance. My above example was based off of the flat top.
Link Posted: 8/4/2005 5:03:46 PM EDT
They have 2 different size spacers. The shorter one sets the witness at "absolute" and the taller one does the lower 3rd. I'm kinda new to the AR platform and ARMS products so I have no idea if this is something new or not..... I have mine mounted directly on the Bushi M4 flat-top rail.
Link Posted: 8/4/2005 5:07:48 PM EDT
Absolute cowitness has the word absolute in it, so it has to be the right way to do it
Link Posted: 8/5/2005 12:17:14 AM EDT

Originally Posted By Chestwound:
They have 2 different size spacers. The shorter one sets the witness at "absolute" and the taller one does the lower 3rd. I'm kinda new to the AR platform and ARMS products so I have no idea if this is something new or not..... I have mine mounted directly on the Bushi M4 flat-top rail.



Whoops! I guess I should do some research (i.e. pay attention to my own rifle once in a while) before I start talking out of my ass. You are correct. I guess the ARMS full and cantiliver (the one I have) really does get the co-witness in the lower 1/3.

But, now I'm more confused. Everyone started talking about lower 1/3 when the LaRue mounts came out. And I read, or at least I think I did, that the LaRue is taller than the ARMS with the full spacer. Actually it is taller if you compare the Aimpoint 3x mag's in Aimpoint mounts to Aimpoint M/ML's in Larue Mounts. Where the 22m68 full spacer puts the two Aimpoints in line with each other. So, I wonder why I got the distinct impression that the LaRue was the only way to get the lower 1/3 co-witness? Uh... Oh well. As long as it shoots where I aim it. Sorry about that!
Link Posted: 8/5/2005 3:42:44 AM EDT

Originally Posted By Redhook:
Whoops! I guess I should do some research (i.e. pay attention to my own rifle once in a while) before I start talking out of my ass. You are correct. I guess the ARMS full and cantiliver (the one I have) really does get the co-witness in the lower 1/3.

But, now I'm more confused. Everyone started talking about lower 1/3 when the LaRue mounts came out. And I read, or at least I think I did, that the LaRue is taller than the ARMS with the full spacer. Actually it is taller if you compare the Aimpoint 3x mag's in Aimpoint mounts to Aimpoint M/ML's in Larue Mounts. Where the 22m68 full spacer puts the two Aimpoints in line with each other. So, I wonder why I got the distinct impression that the LaRue was the only way to get the lower 1/3 co-witness? Uh... Oh well. As long as it shoots where I aim it. Sorry about that!



You're correct, the ARMS full and cantilever spacers put the irons in the lower 1/3. In my experience the Larue mount puts the irons in the lower 1/4 or so. You see people jumping at the Larue, since it's the "latest and greatest", like ARMS was once. Granted, Larue makes excellent products and I can see why new buyers go to them instead of ARMS. I for one am very satisfied with my ARMS mount and see no need to change it to a Larue. If I get another Aimpoint, then I will concider all options.
Link Posted: 8/5/2005 8:20:00 AM EDT
Link Posted: 8/5/2005 9:31:36 AM EDT
[Last Edit: 8/5/2005 9:33:28 AM EDT by Humminbird]
I don't know what Dick's definition of "absolute" co-witness is, but what I'm seeing is this. This is an Aimpoint Comp M2 on an ARMS #22M68 with the cantilever spacer. Call it what you want, but looks like 1/3 to me. I used an image processor to define to diameter of the aimpoint FOV and divided it in thirds. It took me all of 1 minute, so save it.



Link Posted: 8/5/2005 9:42:40 AM EDT
Link Posted: 8/5/2005 10:07:12 AM EDT

Originally Posted By C4iGrant:
Dick defines (hate to speak for him) absolute co-witness as in the middle of the window. The pic that you show is nice, but really doesn't prove anything. I can take a pic of my LT mount and Aimpoint and position it so that it looks like it is in the middle of the window.
C4



No you can't, if you are trying to picture co-witness. You very well can't "position" the irons anywhere you like, now can you? Co-witness is when you have the iron sights PERFECTLY aligned, i.e. the front sight post is centered in the rear aparture, like in the picture above. That is co-witnessing. Move the camera in ANY direction, and it's not.

Take a picture with the LT mount where the irons are as perfectly aligned as above and in the center of the view, and I'll believe you.

BTW, do you really trust Dick's judgement over your own eyes?
Link Posted: 8/5/2005 10:10:17 AM EDT
[Last Edit: 8/5/2005 10:11:17 AM EDT by Humminbird]
Just occured to me that Dick might be using a different height rear BUIS when looking through the sights. I've used the ARMS #40 and the Troy and both provide the same 1/3 sight picture. I'm not sure how it would look through say a KAC 600m BUIS or the Matech.
Link Posted: 8/5/2005 10:11:21 AM EDT
[Last Edit: 8/5/2005 10:28:27 AM EDT by ClarenceDammit]

Originally Posted By C4iGrant:
Dick defines (hate to speak for him) absolute co-witness as in the middle of the window. The pic that you show is nice, but really doesn't prove anything. I can take a pic of my LT mount and Aimpoint and position it so that it looks like it is in the middle of the window.



C4



Can you post that pic to this thread?

I did almost the exact same thing as Humminbird when this topic came up several months ago, where I took the photos other posters had provided of the sight pictures provided by the ARMS and Larue mounts and used a photo editor to quantify where the iron sight axis occurs in the Aimpoint window. I asked for criticism but no one replied with such. Maybe it's still on the archive server? Anyway, I would like to see why Humminbird and I are wrong to reach the same conclusion, and with evidence or argument that goes beyond "don't believe your lyin' eyes!" or "cause Dick said so."
Link Posted: 8/5/2005 10:16:42 AM EDT
Link Posted: 8/5/2005 10:17:42 AM EDT

Originally Posted By Humminbird:
Just occured to me that Dick might be using a different height rear BUIS when looking through the sights. I've used the ARMS #40 and the Troy and both provide the same 1/3 sight picture. I'm not sure how it would look through say a KAC 600m BUIS or the Matech.



All rear sights are the same height when zeroed for the same distance (and I'd bet 3rdtk would use an ARMS not a KAC product). The only you get a higher sight is when you have an adjustable rear (i.e. KAC 600) and have it adjust for a longer range (like 600m).

I'd note in the photo above the front sight post IS NOT centered in the rear aperture. It's close, but not quite there. So I don't think it really proves/disproves either position.
Link Posted: 8/5/2005 10:27:30 AM EDT
[Last Edit: 8/5/2005 10:35:08 AM EDT by Humminbird]

Originally Posted By Forest:
All rear sights are the same height when zeroed for the same distance (and I'd bet 3rdtk would use an ARMS not a KAC product). The only you get a higher sight is when you have an adjustable rear (i.e. KAC 600) and have it adjust for a longer range (like 600m).



Good point.


Originally Posted By Forest:
I'd note in the photo above the front sight post IS NOT centered in the rear aperture. It's close, but not quite there. So I don't think it really proves/disproves either position.



Part of it is covered in the bloom of the dot, but looking at the ears of the front sight base I would say it's as close as it can humanly get. The dot is also centered in the aparture, which should also tell something. How close does it need to be, in your opinion, to prove something? Just curious, since moving the FSP just ever so slightly in the rear aparture view makes virtually no difference how it looks in relation to the Aimpoint FOV, because the rear sight is very close to the eye compared to the other two sights.
Link Posted: 8/5/2005 10:47:13 AM EDT
In an "absolute co-witness" scenario the rear sight aparture should be centered with the Aimpoint frame, but in my photo you can see how there is a crescent moon shape above the rear sight aparture formed by the Aimpoint window.

Grant, I still don't get what you mean by aligning the camera or shouldering the weapon differenly. A line of sight is just that, a line formed by the rear aparture and front sight post. You are either on the line or off it. What you can do is look through the aparture differently, but since it's so close to the eye, it makes very little difference.

I don't know how to explain this any better. If someone with the ARMS mount + full spacer is seeing an absolute co-witness instead of what I've shown above, I would very much like to see it.
Link Posted: 8/5/2005 11:06:48 AM EDT
May I ask WHY anyone feels the need to argue about this shit?
Link Posted: 8/5/2005 11:16:39 AM EDT
Link Posted: 8/5/2005 11:17:34 AM EDT
Link Posted: 8/5/2005 11:23:26 AM EDT

Originally Posted By C4iGrant:

Originally Posted By Lumpy196:
May I ask WHY anyone feels the need to argue about this shit?



Because I just LOVE the term "perfect co-witness!"


C4




Personally, I think its retarded.


But hey, it gives the hens something to cluck about on the net.
Link Posted: 8/5/2005 1:37:15 PM EDT

Originally Posted By Lumpy196:
May I ask WHY anyone feels the need to argue about this shit?



Because it's a simple matter of getting the correct information to people who consider this a factor in their purchasing decision?

But hey, no one's forcing you to read this. Feel free to ignore it and go participate in the latest Aimpoint vs. Eotech thread or whatever non-retarded threads that have your approval.
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