Re: magnifier for vortex?
If I'm not mistaken, the mount that comes with the CompM4 provides an absolute co-witness with the front post. The swing mount that Vortex bundles with the VM3X (made by ADI) will put the magnifier in line with the optic, but you may be looking at interference with the BUIS when you swing it.
The Vortex VM3X has adjustments to center the dot in the magnifier, so a perfect alignment is not necessary. Try not to lose the little adjustment key. No reason...
Mark H.