I have a 15A4CBA2K flat top with the standard front sight. I have been interested in mounting a red dot optic such an eotech 512 with buis. Frankly, I am scared to purchase anything. The whole sight height and co-witness issue has me confused. I have several questions; Has anyone here mounted an Eotech on an M15? Will the red dot co-witness? What rear buis do you use. Thanks in advance!
Nothing to be scared of. All the .223 rifles are basically the same. You can get whatever rear BUIS you would like and it will work fine. A 512 will give you an absolute cowitness which means your irons will line up with the reticle in the middle of the sight window. Some people don't like the look of their FSB in the sight picture so they get a riser for their Eotech or buy the Eotech with the riser built into it. This puts the irons in the lower third of the sight window, thus the lower 1/3 cowitness.
I have my EOtech mounted on a RRA Dominator-2. Built-in rear iron, works great. The dot inside of the reticle sits right on top of the front sight post.
Thanks for responding. I really like the idea of having the iron sights in the lower 1/3 . I was not aware that was even possible. Thanks for the advice.
I was thinking about getting the eotech 512 and a riser today until I saw that the eotech 557 has a built in riser. Does anyone know if the 557 puts the irons in the 1/3 of the optic?
Forgetting about cowitnessing for a moment - one caveat, the ArmaLite A2 front sight could give you problems zeroing with a third party milspec height rear sight. There is an easy fix though, Bushmaster sells a 0.04" taller front sight post for about $6 that will match up with the taller milspec rear sight. If you buy ArmaLite then this should be a non-issue.
Annoying, but we can actually blame Colt for this inconvenience.
I wrote Armalite and asked which rear sight they recommend,and they answered me back very quickly. I believe it was a gentleman named Tim who responded that the MI EX3406 would line up properly with the front sight. So, that's one problem out of the way, I think! Now , I have to figure out which optic to use, preferably an Eotech. I initially wanted the iron sights to co-witness with the eotech, but, another poster mentioned having your irons line up in the bottom 1/3 of the optic. After giving it some thought today,that is the way I want to go
My Armalite has had about 1000 trouble free rounds this year. The only complaints I have is finding the proper bells and whistles for it seem to be a hassle. It's probably just my ignorance showing. Back during my Army days in the early 90's, all we had was a permanent carry handle and simple sling, there are a lot more options to contend with nowadays!
Thanks for your help guys!
The issue with the front sight is that a standard FSB is too low. You need an "F" marked FSB to align properly with a rear BUIS or the taller front sight post from Bushmaster should work and allow any rear sight to be used.
There are several models of Eoechs that have the built in riser. Check out their webiste and take notice of the models that point out the "raised 7mm base".
Since you have an ArmaLite rear sight you should be good to go, AWDeanSr. The problem arises when someone replaces the ArmaLite rear sight with a third party BUIS, which most likely will be milspec height. Anyway, the taller post is an easy fix once you know about it. I'm sure ArmaLite's customer support passes on this little gem of wisdom often.
And cowitnessing is a good way to go, whether you raise the optic slightly for the bottom 1/3 cowitness or go with absolute CW straight through the center of the optic. Bottom 1/3 does get the irons out of the sight picture slightly plus you can go down to pick up the CW or just look over the irons for the red dot.