Some time ago I was out shooting my AR-15. It is an older model similar to the A2, including the iron sights, but I had fitted a red dot optic atop the "carry handle".
I had a fellow shooter ask, "Can I shoot that?" I agreed and handed her the rifle. She was and is a good shot. She knows guns and ARs in particular, but this was her first attempt with a red dot.
The target was a full-size silhouette at about 100 yards... and completely missed. That's near impossible given her skill level.
I asked, "Where did you put the red dot."
She answered, "What red dot?"
She never saw a red dot! I hadn’t explained the new sight, so her initial sight picture just centered the front sight in the rear optic, just like she was used to using with iron sights. Given the set up, she inadvertently missed well above the target without realizing it.
Lesson learned: Don't assume!
Good lesson. I’ve seen this play out often with military personnel. Part of the confusion stems from rifles with red dot optics that are also fitted with iron sights.
Some illustrations. Begin by getting a bore sight:
A red dot optic is its own sight and has nothing to do with any iron sights if they happen to be on the rifle. You’d think this is obvious, but not always. A rifle with no iron sights will have a sight picture similar to this.
A rifle also fitted with iron sights, as most military rifles continue to have, will have them appear in the sight picture. The red dot may appear right above the front iron sight inside the field of view:
Or it may not:
Provided the optic is properly zeroed, all of these are fine.
Cowitnessing is when both the zeroed and aligned iron sights and the zeroed optic both share a sight picture on the same plane. With both sights sharing the same line of sight above the bore and zeroed for the same distance, this can be done but isn’t necessary.
When both sights are zeroed for the same distance and assuming they're the same height above the bore, it makes sense that they’ll aim in at the same place. One way to expedite bore sighting a new optic is to “lollipop” the red dot by adjusting to the top of the front sight while observing both through the rear iron sight. The optic’s zero needs to be confirmed but this should at least get the first shot on paper. Many soldiers have confused this bore sighting expedient as the way the sight is to be used in normal use.
As the introduction demonstrates, the thing to avoid is confusing the use of the iron sights with a reticle-less optic: