I beg to differ. You can keep both eyes open and probably should as closing one eye increases strain on the muscles of the aiming eye.
Hum , I just pulled out my AR with the iron sights . I have to say at close range and well lit area I can keep both eyes open and see the target . How ever low light or longer ranges its much more difficult .
That was just glancing and or looking down the sights . Im not sure if I could keep both eyes open if I were actually shooting . With the guns recoil and then me having to reaquire the target I think that would be very tuff for me . It is easier with the large aperture then the small one but I guess that goes with out saying .
I will tell you this . The next time im out shooting I will be trying this .