What is co-witnessing? You may have heard this term mentioned in various gun forums and Facebook groups. It is a term that means an optic is in line with the iron sights on a firearm. So that when you look through your iron sights your optic reticle should line up with the irons when you aim.
Why would you want to do this? If you have a good red dot that is rugged and has a great battery life, like the highly praised Aimpoint T1, then you might not need iron sights on your gun. However having iron sights on your gun are great in the rare chances that your optic battery dies or even worse the optic breaks and you cannot see through the optic anymore.
But why does it matter that the irons and optic all line up? Ease of use is one feature. Bringing up your gun and aiming it, you get used to seeing the same sight picture. If you have folding iron sights you can leave them down and then if the battery dies, you just flip up the iron sights. That way you can still see through the optic to see your front sight post and aim.
There are other benefits besides having a back up system for your rifle. If you have a good red dot with clear reticle, like a 2moa dot, then the dot will help you be more precise than your ordinary front sight post. You can even zero the irons to a different distance than your red dot. By having two different zeroes, one for your irons and one for your red dot, you can actually accurately engage targets a varying distances. For example, you could set up your irons for a battel zero of 50 yards. The ballistics of 5.56 say that a 50 yard zero is the same for a 200 yard zero. So now you can engage 200 yard targets. Well what about the red dot? You could zero it for a 300 or 400 yards.
You could also use your irons as a quick zeroing benchmark for when you get a new red dot. That is assuming your iron sights are zeroed in already. If so then you just mount your co-witnessing red dot and adjust the reticle to line up with the front sight post. This comes in handy if you invest in a good red dot and swap it onto different guns. You can quickly re-zero the red dot to the each gun you mount it onto.
Now there are some things you need to know about co-witnessing an optic. This really only works with red dots. Your traditional magnified scopes will not work simply because you cannot clearly see your front sight. That does not mean you shouldn't have back up iron sights they just wont be able to work with the scope simultaneously. Unless you go with angled iron sights and shift the sight plane to the side of your gun.
When co-witnessing a red dot with iron sights, the height of the red dot will determine what type of co-witness you have. Absolute Co-witness is when you look through your iron sights, they line up in the middle of the eye piece and block most of the sight picture. Not an issue if your iron sights can fold. It helps to at least have a folding rear sight if you have a fixed front sight on your rifle. Some red dots sit higher on the gun and you can get a lower 1/3 co-witness where your iron sights only occupt the lower third of the sight picture thereby obscurring less of your view.
There are some guns that you might not be able to co-witness due to the design and location of the iron sights. Such is the case for man of the H&K rifles. MP5 style weapons have iron sights that sit very low on the upper receiver. Other H&K guns like the G3, G36, and UMP all suffer from this problem. The rail for an optic sits too high in relation to the iron sights that it is impossible to co-witness the iron sights with a regular red dot sight.
In the case of the CZ-USA Scorpion Evo3, the factory sights sit rather low. An aimpoint or similar red dot will not co-witness unless you mount it directly to the rail. Smaller pistol red dot sights like the Burris Fastfire or Trijicon RMR rail mounted co-witness nicely with the factory sights on the Scorpion.
In the case of handguns you can have iron sights co-witness a slide mounted red dot. Guns like the FNH FNX-45 Tactical and the Smith & Wesson M&P C.O.R.E. all have tall suppressor sights mounted to the slide. Then a pistol mini red dot sight is counter sunk into the slide. By raising the irons and lowering the optic, you can co-witness a red dot on a hand gun. by having your irons co-witness thru a slide mounted red dot, you avoid "chasing the dot". The iron sights give you a reference point when you bring your gun up and sight the irons. When the front and rear sight are aligned the red dot you be right there. Without irons often shooters spend time trying to move the pistol around to see the dot. The other added benefit is if the battery in the red dot dies, you can still aim with the iron sights.