So obviously I am new. I have never hear of this before . Please explain or direct me to info. already available

First off, weelcome.  Second, a cowitness is when you can not only sight down your red dot sight, but you can also look through your red dot to see your iron sights.  The both sight the same way, inthe same area (not offset).  They co-witness.

There is also what's called a 'lower 1/3 cowitness'. That's where the red dot is a bit higher than the iron sights. The iron sights align approx 1/3 high in the view of the red dot sight.

With a red dot optic like an eotech, co witness refers to looking through both the irons and the red dot and both line up; the "red dot" dot will sit on top of your front sight post.

Full co witness is when the red dot is low mounted and the irons show up in about the center of the red dots field of view.

1/3 co witness is when the red dot is higher mounted and the irons show up in the lower portion.

Both are the same idea, just different in where the irons are in view.

For me the idea is you can verify the adjustment.  If you want to check that your red dot is right, flip the irons up and look through both.  Also, if you have a co witness and your optic goes down, flip the sights up and you know your POA/POI is the same as when you were using the red dot.  It is a simple way of confirming your zero hasn't been screwed with and using your irons as back ups to the optic.

Now, in contrast if you use a red dot mounted on a carry handle the irons and red dot cannot co witness (assuming it's not a goose neck mount).  Both have to be sighted in independently.  If one is messed up you would never know until you shoot.

(The others answered, while I was typing - same thing)

yes I have read about this here. but the ? is why? do some find this to be so important.

thank you all for the explanation, and thank you for the welcome.
just waiting for m1s to send the kit.

RLR350 I think pretty well covered it! Co-witnessing is good because your optic and iron sights are hitting on the same spot. Should your optic experience a malfunction or completely stop working you can use your iron sights and you are back in business! The two work together even when they aren't used together! Many law enforcement agencies require that their patrol rifles, if used with a red dot scope or optic, must co-witness together. Think of it as a back-up plan!!!

I have read about optic mounts that allow the use of iron site under/through on  an a2. Any thoughts or opinions??

That requires a different cheek weld, or more accurately no cheek weld at all on the optic mounted on the carry handle.

I co-witness, but I use fold downs. What gets me are the guys w/ perm. irons who want co-witness. All it does is block out target area for me... no thanks.

i have an lmt fixed sight with a larue lt150 aimpoint mount, this gives you a lower 1/3 cowitness. you can put the dot on the post using the rear sight, or by tilting the rifle slightly the dot appears above the irons altogether with no sight obscuring the dot. the navy seals ran this setup with the mk18 mod0, only they use the wilcox mount for the aimpoint.  with this setup you get the 100% reliablity of the lmt fixed rear sight.



http://www.navy.mil/management/photodb/photos/060209-N-4374S-008.jpg

I don't mean to say that it will block out the dot, I mean it blocks out target area. Why not just use the dot and have a better visual picture, then use fold-ups if you ever need back-ups.

But then again, I'm not a Navy Seal.

I have my red dot sighted in at 100 yds and my irons sighted in for 200 yrds with lower 1/3 co-witness. Started out as a theory, but in reality it works quite well for me, if it's close I use the reddot, and for longer ranges I can 'drop down' to my irons.

Interesting, I've never thought of having the 2 systems sighted at different ranges, but it does makes some sense.

If iron sights are blocking the target area then you are doing it wrong.  Keep both eyes open and you will be able to clearly see both the target area and the red dot even if you leave the front lens cap closed.

I took it that he meant that the front sight post blocks some of the field of view when looking through the red dot sight.  If you put the dot on a bad guy's chest, the front sight post would block out part of the person's body.  I believe that is why he posted that he uses fold down sights so he can fold the sight down out of the way, giving more of a field of view.

Is there some other way of co-witnessing (besides 1/3 co witness) that allows the front sight to be fixed in the "up" position and not block out any field of view?

I get that you are saying that if both eyes are open, you will see the complete target with your open, non-sighting eye.

Hopefully that made sence.

You get me.

I noticed it last time I was hunting. Small buck was across a small/medium ditch. Put my red dot on him.. he went behind a tree, so I dropped my gun to get better FOV again. When I did, I noticed a much larger rack sticking up from the ditch. Had I not dropped slightly, the FSB would have blocked it out completely and I wouldn't have seen the bigger animal.

I would think for combat awareness you'd want maximum FOV at all times. Yes, your left eye can compensate, but blockage is blockage. And for longer shots (could be due to my crappy eyes) I like to squint my left eye.

How to improve your shooting & How to shoot with a red dot sight