This is his first lower third RDS & he doesn't like how it obstructs the view of his FSB when using his irons.
At least that's what I got out of it.
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I have a red dot on my issued rifle.. I'm pretty sure i know exactly how it is meant to be used. I just want to be able to see my iron sights because i still shoot with them to retain that skill...I'm not so much worried about the Eotech lining up with them. I turn it off and shoot irons. but i cannot see my front sight through the Eotech body..
I'm not up on the 500 series EOTechs (just noticed you stated you have a 556). I think those have NV. You may want to sell it & look into getting an XPS3 model that has absolute co-witness with NV.
If you have properly zeroed your irons & the dot - when you line up the irons you will have a co-witness. I prefer an absolute with flip up sights. I want to look through the sights & see that dot sitting atop the front post to verify that both are still zeroed, then I flip those damn things down for a clean unobstructed view & just use the dot. If the dot craps out flip them back up & have at it.
I don't quite understand the interrogation you got for wanting to be able to flip up your irons and use them through your Eotech to maintain skill or in the event of the Eotech going down. I could see the rear aperture being cut off hindering this quite a bit.
I will say, I just got an EXPS2, which I think is pretty much the same height since it is also designed for a lower third cowitness and it does a nice job with my Magpul MBUS Pros. It doesn't cut into the circle at all. I wonder if the KAC sight is sitting a little lower due to its rear adjustment. If you have access, maybe you could try a different set of back up sights and see how they line up.