For my 300BO AR, I used the lower 1/3 co-witness mount for my Trijicon MRO optic. For my AR-V, I went with that again without really thinking about it.
I’m having second thoughts now because of the 9mm bullet trajectory - I’m thinking now that the red dot should be mounted very low. Anyone have any suggestions or thoughts on the matter?
If the mount is very low, the MBUS might get in the way? Maybe just leave them off but keep handy for emergency? Thanks for any input.
I’m doing lower 1/3rd on mine. It’s not so much a matter of height as it is knowing your distances. A red dot that is on a 1.93" mount, lower 1/3rd, absolute, or even no mount will all still zero at any distance (such as 50 yards). But where it goes from there depends on your bullet, barrel length, etc. The mount helps you get the right sight picture for you.
Bought mine today and had the same thought. I want something mounted low but what about the back ups? What did you figure out?
Ended up keeping the mount I bought since it was “in-hand” 
but will probably get a shorter one eventually. With a non-rifle bullet coming out of my 8" barrel, I’ll have a muzzle velocity of about approx 1300 fps. From there, you just have to know the flight path to make adjustments!
For the flight path, the following is something you can put your bullet/muzzle velocity information in (my data, however bad, should be filled in already) and it’s easily modified. Once you get your data in, just click the “Create Graph” button and scroll down:
9mm Trajectory
I ended up sighting in at about 15 yards but will re-zero at 25 yards for a “flatter” trajectory since I won’t be doing much past 50 yards anyway.
Let me know if you have any questions about this - everything is a variable, from muzzle velocity, sight height, etc.
Don’t go so low that you have no cheek weld. Optics can be adjusted for height of dot for this reason.
BTW, just noticed that this is your first post, so welcome to the forum!
Appreciate it. I’m glad this community is here. Not a whole lot of info out there on the ar-v besides the normal unboxing review stuff. I’m excited to get this thing setup
This original post is a bit old, but I’ll offer my (brief) experience anyhow. On my AR-V (10.5 in barrel) I mounted a SIG MSR which I pulled off another gun. The MSR is designed to cowitness with iron sights. I zeroed at 30 yds and ran it on the club’s carbine challenge course. Targets on the course range from 50 to 12 yds and I didn’t need to adjust my point of aim to make hits at the different distances. I like the higher mount because it is the same as I use on my 5.56 and it worked for me.
That’s a great, practical description of what you did. 30-yard zero has a great trajectory for your usage, and it doesn’t stray too far from +/- 1-inch between 15 to 70+ yards.
@GuitarGuy had a great point about not going so low that you don’t have a good cheek weld. Once I thought about that, it saved me from getting a really low mount and having to kink my neck to get a good sight picture!