With a red dot sight, most shooters can engage targets with both eyes open at short ranges and often only use the dominant eye for longer distances despite eye relief.
A red dot will have little to no eye relief (should be zero) with a 1X optic and very little parallax (yep, even Aimpoints have parallax). Red dots are usually available with a 2MOA or 4MOA dot size (2MOA is more popular these days as it only covers 2" at 100yds). Typical Red Dots on the AR platform ride in mounts that have an absolute co-witness or a lower 1/3 of the optic co-witness with standard height iron sights mounted on the rail (flip-up or fixed).
Due to optic mount height over bore (optic center line to rail height), you would hold higher for closer ranged targets.
For instance, if you zero at 50yds with a 5.56mm NATO platform, you will probably need to hold about 1" high on targets 25yds and less with the distance decreasing down to 10yds somewhere just less than 2"--scale accordingly for the .22LR. If you are shooting at a man silhouette less than 10yds away with a 50yd BZO, you would need to aim into the hairline to shoot into the brain box (~2") and likewise to hit the center of the 8" circle in the body.
Standard optic center line to rail height for a 30mm ring optic mount is:
- 1/3 cowitness ~1.69"
- Absolute co-witness ~1.52"
Flip mounts for magnifiers are very nice--they allow the shooter magnify at range, flip over out of the way for close-in work. American Defense Mfg and LaRue Tactical make some of the best Aimpoint, EOTech, and magnifier mounts. A 3x magnifier will certainly help behind a 2MOA or 4MOA dot and for those of us with astigmatism, the magnifier will sharpen the dot and remove the fuzzy edges our irregularly shaped corneas produce for us. The use of a magnifier will decrease your eye relief significantly from a red dot only configuration and you will probably enjoy a cantilever mount on the optic to push it out forward of the magnifier but not too far down the rail from your magnifier, which will be dictated by your cheek weld for eye relief. Adding a magnifier will significantly add weight to your platform is that is a concern, but an Aimpoint 3X magnifier is worth its weight in gold.
I run the Vortex Sparc on my 15-22 with the 2x magnifier. For whatever that is worth...
This is a picture of my custom built AR-15 with Aimpoint optic/mag in LaRue mounts.