View Full Version : RDS: Co-Witness or Lower 1/3 Co-Witness... your thoughts?
As I consider purchase of a riser for a Primary Arms Advanced Micro Dot RDS, the question arises... co-witness or lower 1/3 co-witness?
Thus, I am here, once again, to consult the (collective) Oracle. So, what are your thoughts?
Personal preference based upon cheekweld and BUIS through the optic. I like 1/3 but YMMV.
Tomac
It's preference, but I do think rifle requirements should play some role in consideration.
I use fixed irons and a lower-1/3rd co-witness, QD mount on my HD rifle. If the dot dies for whatever reason, I can either use my irons immediately, or if I've got time, take the dot off. Also, I tend to treat my rifles as irons-first, and a red dot needs to be set up so that I can go back to irons immediately. I do the same with AK's... lower-1/3rd co-witness, and a QD mount.
Absolute with fixed irons can be cluttered, but if you've got flip-ups, it is more about cheek weld. There's a minimal, but noticeable, difference in cheek weld between absolute and lower-1/3rd. If you're running with a fixed mount, make sure you re-zero your irons through the glass. If you're going QD, zero without the glass in between. The glass can distort your sight picture.
Personal preference based upon cheekweld and BUIS through the optic. I like 1/3 but YMMV.
Tomac
I kind of have to agree here. It is preference. Have used both, I have my preferences to 1/3, but I know others who prefer absolute. Try both if you can and see what you like the most.
I prefer lower 1/3. The iron sights obstruct less of the view through the optic.
malstew123
11-29-15, 20:57
I prefer is a lower 1/3 mount with fixed front and rear irons. I learned to shoot with irons so I instinctively want to get the nose to charging handle cheek weld every time, now all I do is bring my head up just slightly for the red dot or down for just the irons.
Experience has slowly taught me that a greater clear field of view is better in practically any situation. Lower 1/3.
Just like everyone else so far, I prefer lower 1/3. Less clutter when looking through the optic is a good thing.
Lower 1/3. More forgiving of head position, less fatiguing when shooting a lot.
With a lower 1/3, the red dot sits higher, correct? If so, would like that, as I find the iron BU sights seem low on the rifle to me and I have to scrunch my head lower on the butt stock than I would prefer with the irons. If the red dot sits higher, I wouldn't have to scrunch my head down so low on the stock.
With a lower 1/3, the red dot sits higher, correct? If so, would like that, as I find the iron BU sights seem low on the rifle to me and I have to scrunch my head lower on the butt stock than I would prefer with the irons. If the red dot sits higher, I wouldn't have to scrunch my head down so low on the stock.
The red dot appears centered in the glass with the iron sights out of the way when viewing the optic through the middle. When you lower your eyes and look through the irons sights, the red dot appears to be at the top of the front sight. If you are going to use the red dot as your primary aiming device, I think it is a better way to set up the carbine for most people.
Another for lower 1/3 for all the reasons already stated.
Auto-X Fil
11-30-15, 12:17
I use absolute with flip-up sights, and lower 1/3 with a fixed FSB and folding rear. For me and the low-profile stocks I use, the stock irons height is about perfect. But, I can make lower 1/3 work well, and it's worth it for me to get the dot up off the post.
BrigandTwoFour
11-30-15, 13:26
If you have a fixed front, then I would prefer a lower 1/3. However, if you have folding front and rears, then it really depends on your preferences.
Most of my shooting these days falls into the category of "traditional marksmanship" rather than high speed dynamic stuff (10 rd limits and bullet buttons kinda put a damper on that). I find that a lower 1/3 works well for "head up" fast shooting, but the requisite raised head position gets mighty uncomfortable for me when shooting from positions such as prone for long periods of time. I found the same thing with my TR-24 and an ADM Recon-H mount a couple years ago.
In short, I like to keep my optics mounted at the same level as the iron sights, unless the iron sight will obstruct my sight picture through the optic.
titsonritz
11-30-15, 14:02
I use 1/3 co-witness. My front iron in non-folding, my rear is folding. The video below does a good job of explaining the differences along with the pros and cons of each.
https://www.youtube.com/watch?v=ogX-cIx9hpk
I use 1/3 co-witness. My front iron in non-folding, my rear is folding. The video below does a good job of explaining the differences along with the pros and cons of each.
https://www.youtube.com/watch?v=ogX-cIx9hpk
titsonritz... thanks for the video link.
My BUIS are flip up. I'm thinking I would probably keep them flipped down and use just the RDS most of the time. Lower 1/3 co-Witness sounds like a good way to go.
I use folding front and rear with lower 1/3. Works great.
Mysteryman
12-01-15, 03:09
Experience has slowly taught me that a greater clear field of view is better in practically any situation. Lower 1/3.
FOV with an RDS is the same regardless of wat is in front of your sighting eye, as long as you keep BOTH eyes open and focus on the target, not the dot. The FSB becomes a ghosted image that is blended with the unobstructed image coming from your non sighting eye.
MM
FOV with an RDS is the same regardless of wat is in front of your sighting eye, as long as you keep BOTH eyes open and focus on the target, not the dot. The FSB becomes a ghosted image that is blended with the unobstructed image coming from your non sighting eye.
MM
I'm not referring to the listed field of view. Thanks though, I'm sure someone who's never looked through an RDS will find your comment helpful.
Mysteryman
12-01-15, 16:54
I'm not referring to the listed field of view. Thanks though, I'm sure someone who's never looked through an RDS will find your comment helpful.
Neither was I. FOV is FOV. There is no obstruction with a reddot when used properly with both eyes open. The desire for a "greater FOV" with a reddot is a gimmick and a sure sign that the individual doesn't fully understand how a reddot is to be employed. The FSB or any other attachment is a non issue as it becomes a ghosted image. The Armson OEG is an excellent example(and the first reddot/rapid aiming point type sight) of how binocular/stereoscopic vision works. It has no lenses and absolutely zero FOV when viewed by the user. the FOV is generated by the non sighting eye which shares that image with the sighting eye. The OEG housing, or the reddot housing becomes ghosted and a non issue as far as FOV is concerned. The same effect occurs when you hold a paper towel tube to your eye then place your hand against it, it appears as though you have a hole in your hand thanks to stereoscopic vision and image sharing between both eyes.
MM
Neither was I. FOV is FOV. There is no obstruction with a reddot when used properly with both eyes open. The desire for a "greater FOV" with a reddot is a gimmick and a sure sign that the individual doesn't fully understand how a reddot is to be employed. The FSB or any other attachment is a non issue as it becomes a ghosted image. The Armson OEG is an excellent example(and the first reddot/rapid aiming point type sight) of how binocular/stereoscopic vision works. It has no lenses and absolutely zero FOV when viewed by the user. the FOV is generated by the non sighting eye which shares that image with the sighting eye. The OEG housing, or the reddot housing becomes ghosted and a non issue as far as FOV is concerned. The same effect occurs when you hold a paper towel tube to your eye then place your hand against it, it appears as though you have a hole in your hand thanks to stereoscopic vision and image sharing between both eyes.
MM
I was probably too vague in my first post.
Reel back a little bit with the regurgitation and find where I wrote about the irons obstructing the view. I didn't. This about cowitness and when it actually comes into play - when you're using the irons, and at that point your RDS becomes an artifact in your field of vision. Not much, but enough, and a 1/3 cowitness moves the hood of the RDS a little further away.
You affirm this yourself referring to ghosted images.
Definitely absolute co-witness and folding rear sights for me. I like cheek weld. Holding your head up off the stock with your neck muscles gets tiresome and negatively impacts accuracy, especially in prone and seated. Even if I had a rifle that was only ever used for close quarters, I'd like solid cheek weld. Having solid fundamentals with your main optic is more important to me than being able to transition to your backup irons 1 second faster in the long shot event that something happens to it. I agree with Mysteryman that a FSP is a complete non-issue. You don't even notice it when shooting fast at close quarters.
Boba Fett v2
12-01-15, 22:38
I prefer lower 1/3 for all the reasons stated.
4thPointOfContact
12-01-15, 22:44
Personal preference as everyone says but.....
Fixed sights? definitely lower 1/3rd
Folding sights? My pref is for absolute and keeping a consistent cheek weld regardless of which sights I'm using
I go with no witness.
Do not want to see iron sight at all when using RDS. So I use flip up sights. When flipped up, the are co-witness so cheek weld is identical.
Mysteryman
12-02-15, 17:41
I was probably too vague in my first post.
Reel back a little bit with the regurgitation and find where I wrote about the irons obstructing the view. I didn't. This about cowitness and when it actually comes into play - when you're using the irons, and at that point your RDS becomes an artifact in your field of vision. Not much, but enough, and a 1/3 cowitness moves the hood of the RDS a little further away.
You affirm this yourself referring to ghosted images.
Still not an issue. You don't need to look any further north than the top of the front sight post. With an absolute cowitness you still have half of the objective sense diameter to use. Ideally you should be shooting irons with both eyes open as well but that is a tall order for most of us. Identifying your target then bringing the sights into your line of sight completely negates the "cluttered" FOV through the reddot. Your focus is on the front sight anyway so the rest of the FOV is irrelevant.
MM
Mysteryman
12-02-15, 17:43
I go with no witness.
Do not want to see iron sight at all when using RDS. So I use flip up sights. When flipped up, the are co-witness so cheek weld is identical.
That's still an absolute cowitness, whether you run with the irons up or down. ;)
MM
That's still an absolute cowitness, whether you run with the irons up or down. ;)
MM
Not really as I never use the two together. Thus differentiating from an FSB/M4 vs folding.
Mysteryman
12-03-15, 16:05
Not really as I never use the two together. Thus differentiating from an FSB/M4 vs folding.
You don't need to use them together to be a co witness. The sight height for both your optic and irons are on the same plain, they are co witnessed. You don't need to adjust your head or cheek weld to use either.
MM
You don't need to use them together to be a co witness. The sight height for both your optic and irons are on the same plain, they are co witnessed. You don't need to adjust your head or cheek weld to use either.
MM
whatever
Mysteryman
12-04-15, 00:30
whatever
Not trying to be a dick but it isn't a "whatever". Co witnessing or lower 1/3 co witnessing your irons has nothing to do with whether you run them up or not. Co witnessing is about where your iron sights line up with your optic(non magnified that is). Proper terminology is important for clear effective communication.
MM
Not trying to be a dick but it isn't a "whatever". Co witnessing or lower 1/3 co witnessing your irons has nothing to do with whether you run them up or not. Co witnessing is about where your iron sights line up with your optic(non magnified that is). Proper terminology is important for clear effective communication.
MM
You are absolutely correct. With Absolute the BUIS will line up in the center of the optic's tube with the dot OFF. Lower 1/3 the BUIS will line up at the bottom third of the optic's tube with the dot OFF. When you turn the dot on it is a whole other animal as you can get the dot to sit on top of the front sight post regardless of which height mount you go with. The dot will float and still hit POA = POI. Many like the lower 1/3 because there is less of the front sight cluttering up the optic when looking dead center. I happen to be able to block out the front sight tower easily so I like an absolute co-witness as my optic will sit lower on the rail which gives my face a better cheek weld. With a 1/3 mount I am bordering on a chin weld.
WatchTheWorldBern
12-08-15, 17:31
Can anyone with experience running lower 1/3 with front and rear flip ups explain why they choose that over absolute? Doesn't make sense to me (I understand lower 1/3 if you're stuck with a fixed gas block sight), but I've never done it.
The only time 1/3 or absolute really matters for me is when the red dot fails. At that point you are going to need your BUIS. When using my red dot I shoot both eyes open, but when using my sights I use my dominant eye only. With 1/3 I can see more of the area around the target through my RDS giving me better situational awareness. This is a big deal under dynamic and stressful conditions.
It would not do to have an innocent person unexpectedly step into the line of fire and get hurt because your view was limited by having a large part of the sight picture blocked by an absolute cowitness BUIS. With a greater part of the area around your target visible in 1/3 cowitness chances are you are more likely to see that person in time and hold your shot.
sasquatchoslav
12-09-15, 06:32
I'm a huge fan of this approach and being an old fart raised on iron sights exclusively for probably 10 years b/f I started using glass I always like keeping them right 'at my side'. As far as 'weirdo' statuss goes I probably get asked 10x a year 'why do you shoot with your BUIS set popped up?'. Because I switch back and forth b/t scope and BUIS every time at the range. If my optics ever go down it's a seamless transition to BUIS. Got to do this last year when my batts went bye bye and didn't even flinch in the middle of my 30 round mag just kept on pinging.
Mysteryman
12-11-15, 19:06
You are absolutely correct. With Absolute the BUIS will line up in the center of the optic's tube with the dot OFF. Lower 1/3 the BUIS will line up at the bottom third of the optic's tube with the dot OFF. When you turn the dot on it is a whole other animal as you can get the dot to sit on top of the front sight post regardless of which height mount you go with. The dot will float and still hit POA = POI. Many like the lower 1/3 because there is less of the front sight cluttering up the optic when looking dead center. I happen to be able to block out the front sight tower easily so I like an absolute co-witness as my optic will sit lower on the rail which gives my face a better cheek weld. With a 1/3 mount I am bordering on a chin weld.
The only time 1/3 or absolute really matters for me is when the red dot fails. At that point you are going to need your BUIS. When using my red dot I shoot both eyes open, but when using my sights I use my dominant eye only. With 1/3 I can see more of the area around the target through my RDS giving me better situational awareness. This is a big deal under dynamic and stressful conditions.
It would not do to have an innocent person unexpectedly step into the line of fire and get hurt because your view was limited by having a large part of the sight picture blocked by an absolute cowitness BUIS. With a greater part of the area around your target visible in 1/3 cowitness chances are you are more likely to see that person in time and hold your shot.
The FOV argument is utter nonsense. Whether you have a fixed front sight base or not it to makes no difference. The FOV when using a reddot sight(or an OEG for that matter) is exactly the same as what you see while looking at the computer screen. BOTH EYES OPEN means your brain is receiving as much visual stimuli as possible. Having binocular/stereoscopic vision allows us to look "through" the FSB and the reddot housing thanks to the unobstructed view of the non dominant/aiming eye. With both eyes open and your focus on the target, there is absolutely no loss of FOV.
The lower third co witness being less cluttered is also flawed. With your irons in the lower third, the gain in peripheral area when aiming with one eye only(again not ideal but is most often the case) around the front sight is gained ABOVE the sight more than BESIDE the sight by design. The widest point across a circle is called... The diameter, and it is found at the centre of the circle, where it perfectly bisects the shape. It is not found in the lower third. Having an absolute co witness puts the most free space directly opposite both sides of the front sight post. Why you would want or need more viewing area above the front sight post is beyond me. Furthermore, when using iron sights you are to focus on the front sight as the shot is taken, which means you shouldn't in theory ever see that innocent person step into view, so it's a moot point. If you aren't shooting then the rifle shouldn't be mounted as the iron sights offer no advantage for PID of the target or improved precision for the shot. Burying your head behind the irons and leaving it there only serves to diminish your situational awareness even more. Keeping both eyes open and scanning for targets can be accomplished with irons or optics, whether the rifle is mounted or not. Upon finding a target you can rough align the rifle with the target by mounting the rifle with both eyes open(if it isn't already mounted) and then close the non dominant eye to allow for a finite focus on the irons for the shot.
If we are talking about a defensive shoot or offensive shoot, there will be a lot going on, and tunnel vision and flattening of the cornea are some of those things. Neither of which help you remain alert and cognizant of your surroundings. I mention this because I find worrying about the FOV of your irons through your optic and the perceived advantages of the lower 1/3 cowitness over absolute cowitness to be pointless. What I see making more of a difference is the consistent cheek weld and thus sight picture offered by the absolute cowitness. Being able to practice your iron sight skills with the aid of your reddot is a cheap tool that pays dividends every time you use it. With most of us mounting our reddots with some form of QD mount, there is no need to fret over so called "cluttered FOV" as the optic comes off without delay or fuss. I know some will proclaim that there might be a time where you need to shoot NOW and you can't remove your optic. To that I say, **** it, make do with what you have and solve the problem. To plan for such near impossible events is akin to mental masturbation.
MM
sidewaysil80
12-11-15, 19:45
I prefer absolute but will only use folding sights. Thus, they aren't obstructing anything.
The FOV argument is utter nonsense. Whether you have a fixed front sight base or not it to makes no difference. The FOV when using a reddot sight(or an OEG for that matter) is exactly the same as what you see while looking at the computer screen. BOTH EYES OPEN means your brain is receiving as much visual stimuli as possible. Having binocular/stereoscopic vision allows us to look "through" the FSB and the reddot housing thanks to the unobstructed view of the non dominant/aiming eye. With both eyes open and your focus on the target, there is absolutely no loss of FOV.
The lower third co witness being less cluttered is also flawed. With your irons in the lower third, the gain in peripheral area when aiming with one eye only(again not ideal but is most often the case) around the front sight is gained ABOVE the sight more than BESIDE the sight by design. The widest point across a circle is called... The diameter, and it is found at the centre of the circle, where it perfectly bisects the shape. It is not found in the lower third. Having an absolute co witness puts the most free space directly opposite both sides of the front sight post. Why you would want or need more viewing area above the front sight post is beyond me. Furthermore, when using iron sights you are to focus on the front sight as the shot is taken, which means you shouldn't in theory ever see that innocent person step into view, so it's a moot point. If you aren't shooting then the rifle shouldn't be mounted as the iron sights offer no advantage for PID of the target or improved precision for the shot. Burying your head behind the irons and leaving it there only serves to diminish your situational awareness even more. Keeping both eyes open and scanning for targets can be accomplished with irons or optics, whether the rifle is mounted or not. Upon finding a target you can rough align the rifle with the target by mounting the rifle with both eyes open(if it isn't already mounted) and then close the non dominant eye to allow for a finite focus on the irons for the shot.
If we are talking about a defensive shoot or offensive shoot, there will be a lot going on, and tunnel vision and flattening of the cornea are some of those things. Neither of which help you remain alert and cognizant of your surroundings. I mention this because I find worrying about the FOV of your irons through your optic and the perceived advantages of the lower 1/3 cowitness over absolute cowitness to be pointless. What I see making more of a difference is the consistent cheek weld and thus sight picture offered by the absolute cowitness. Being able to practice your iron sight skills with the aid of your reddot is a cheap tool that pays dividends every time you use it. With most of us mounting our reddots with some form of QD mount, there is no need to fret over so called "cluttered FOV" as the optic comes off without delay or fuss. I know some will proclaim that there might be a time where you need to shoot NOW and you can't remove your optic. To that I say, **** it, make do with what you have and solve the problem. To plan for such near impossible events is akin to mental masturbation.
MM
Nobody has made the claim to be shooting with one eye open, and forcing that factor into another's argument, in order to defeat it, isn't very clever.
You're actually making the assertion that a shooter cannot perceive a red dot housing within his view. I don't think that needs a rebuttal. Regardless, this tells me you have in your head the image of looking through an optic very close to the eyes and not one mounted forward on the receiver - so, limited experience.
A little condescension with a smattering of blatant practical ignorance.
Overall, I've had the impression that you find yourself well read on the subject but absolutely bereft of any practical experience on the matter whatsoever - and it doesn't make you seem intelligent.
Mysteryman
12-13-15, 00:16
Nobody has made the claim to be shooting with one eye open, and forcing that factor into another's argument, in order to defeat it, isn't very clever.
Never said anyone made the claim, just telling you what I see frequently both at the range and from online discussions. People are often too stupid to read the f**king manual and use them incorrectly. Then proceed to talk sh*t about reddots when they haven't the faintest clue how they work or how they're to be used. I mention two eye open use because of this. Eliminating confusion on the discussion goes a long way to preventing the need to backtrack.
You're actually making the assertion that a shooter cannot perceive a red dot housing within his view. I don't think that needs a rebuttal. Regardless, this tells me you have in your head the image of looking through an optic very close to the eyes and not one mounted forward on the receiver - so, limited experience.
Thanks for guessing as to my experience and my usage of optics, but you would be wrong. I like my reddot on the far end of the receiver, not molesting my eye like a low dollar optic with near zero eye relief. I never said the optic housing was imperceptible, I'm saying that your view is unobstructed and unaffected by the ghosted image of the housing. Focus on the target, not the dot, and not the sight housing. It's really not that difficult.
A little condescension with a smattering of blatant practical ignorance.
Overall, I've had the impression that you find yourself well read on the subject but absolutely bereft of any practical experience on the matter whatsoever - and it doesn't make you seem intelligent.
In the bold.
MM
Rotorhead84
12-13-15, 16:00
I like lower 1/3rd
misanthropist
12-13-15, 18:50
I like lower third BUT:
Something I never see mentioned anywhere is that some popular RDSs can actually interfere with some popular sights.
For example, I have a T1 on a DD mount and Troy fixed sights. The emitter on the T1, at least if you run it up at the front of the receiver like most do, clearly obscures part of the rear aperture on the Troy if you have it on the large aperture.
It's not a huge issue, but it does distort the shape of the aperture slightly so it's going to alter your POA. On the small aperture it's hard to tell if it's in the way at all, but it's got to be close, for sure. Granted, any effect it has is going to be pretty subtle, but depending on the level of precision you demand from your irons...
Anyway I thought I'd mention it since I've literally never seen it addressed anywhere, in 10+ years of talking ARs on the internet.
^ That! My Eotech EXPS-2 mounted all the way forward did this very slightly with my Magpul MBUS Pro sights. It cut very slightly into the bottom of the aperture keeping it from being circular, even the small one and it ****ed me all up to the point that I could barely get a good enough group to consider them zeroed at 50 yards. The funny thing is that I didn't even notice it until I started shooting it. I ended up moving things around and now those are under a scope and I put fixed irons with the Eotech. The Eotech is now back in Ann Arbor and as I wait for my refund I'm now leaning toward an Aimpoint with absolute cowitness and fold down sights again.
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