The term 'co-witness', when used in reference to red dot sights, indicates how the sight's optical tube aligns with back-up iron sights (BUIS). This is determined by the height of the sight's mount (aka height above bore). There are typically two options, absolute and lower 1/3.
'Absolute' co-witness means that the iron sights, when properly aligned, sit in the center of the sight's optical tube. 'Lower 1/3' co-witness uses a slightly taller mount so that properly aligned back-up iron sights sit slightly lower in the optical tube.
The Key Distinction
With absolute co-witness, the shooter's cheek weld/ position on the rifle is exactly the same when using the red dot sight as it is when using iron sights. Consistency is the main strength of this method. With a lower 1/3 co-witness, the cheek weld is relaxed a bit so that the head and eyes are lifted slightly. Proponents of lower 1/3 argue that this head-up position on the gun means improved awareness.
- Absolute allows for use of the sight tube as an emergency rear sight aperture if the optic fails. This is obviously faster than folding up the rear BUIS.
- Some shooters report that they find themselves inadvertently attempting to center the front sight post in the sight tube, which slows them down.
- Some shooters feel that lower 1/3 provides a 'cleaner' sight picture, since the optical tube isn't obstructed by the BUIS. In theory, proper threat-focused shooting should make that irrelevant (key word 'theory').
So Which is Better?
Which of these two mounting methods is better has been the subject of a lot debate. In our experience, the answer is 'it depends'. Each shooter's experience and anatomy are major factors in determining which is most effective. Shooters that are transitioning from an EOTech or iron sights after many years of using them, for instance, might be more comfortable with absolute co-witness. Shooters with a longer neck may may tend more toward lower 1/3 than shooters with more muscle mass or a shorter neck. There doesn't seem to be a formula- in the end, it's about which one feels and (much more importantly) works better.
There are a lot of good, quality mounts available for Micro sights. We like the Micro Mount with Spacer from Daniel Defense, though, because of its versatility. Rather than having to decide in advance which mount height they prefer, shooters can buy the mount and try both before making a final decision. This is especially helpful when buying for teams or units. The team can have common equipment AND individual operators can choose the option that's most effective for them.