The distance between the red dot and magnifier doesn't matter.
However, the gap between can make the top rail setup look sloppy.
Unless they have misaligned optic heights, how close they are mounted don't really matter.
The only thing the shooter has to adjust are the eye piece ring, elevation & windage dials on the magnifier to sharpen the reticle and center it.
Let's take a look at some configurations below:
Mount Too Close
Mounting the two optics too close isn't a problem if there is enough clearance.
Just make sure to mount the magnifier in the deployed position, and ensure it can be fold/unfold without bumping into each other.
Quick detach magnifier mount makes the adjustment so much easier. Highly recommended over a fixed mount.
Shop for a red dot magnifier here
Mount Too Far
Mounting the magnifier too far behind the red dot isn't a big problem either if the sight picture is clear and there isn't too much of a wasteful gap.
For the setup above with the Vortex Micro 3X and Trijicon RMR, the RMR's American Defense QD lever actually forces the Micro 3X to be mounted 2 rail slots back. So different types of mounts can affect how close they can be mounted on the top rail.
The red dot doesn't have eye relief limitation, however, the magnifier scope does. Please be mindful to have enough eye relief distance based on how you cheek weld the rifle.
It's always okay to move the red dot forward if need to if you're using a long magnifier like the Aimpoint 6X. As long as the optic to eye distance works for you, there isn't much to worry about.
There are forward leaning red dot optic mount available on the market, and the user can shift any ring clamped magnifier (Aimpoint 3X or 6X) a little bit forward.
For example, the charging handle gets in the way of gripping the latch if the magnifier QD level mount is mounted too close to the rear. Try mount them more forward so the shooter's finger doesn't bump into it.
Different optic and magnifier height is a problem if they misalign a lot.
It's okay to have a slight height difference, but not too much.
If you have an absolute co-witness height for the red dot, then an absolute co-witness height for the magnifier. The same goes for 1/3 co-witness optic height.
Why does my reticle looks blurry through the magnifier?
If the reticle looks blurry through the magnifier you should try the followings:
- Correct eye sight or wear glasses for shooters' with astigmatism
- Dial the magnifier's eye piece to focus on the red dot's plane
- Clean red dot or holographic sight's lens
- If reticle still looks blurry, then it means the red dot sight inherently have emitter refraction error, try another one (Check out our best red dot sights guide here on our site)
Does magnifier change the size of the reticle size?
When looking through the scope magnifier, the red dot reticle remains the same size. 1 MOA will still be 1 MOA. 2 MOA will still be 2 MOA.
The only difference is that the dot looks much cleaner and rounder under a magnified scope for shooters with astigmatism.