I've seen an optics mount for pistols that attaches to the Picatinny rail under the barrel, then arches up over the slide so that optics can be mounted. What is the proper nomenclature for this gizmo? It looks like it would be fun to go with my M&P.
They are called optic mounts or scope mounts, the problem is that the need another point of attachment. Either on the trigger guard or frame. I have a B-square mount for a HK that mounts to the rail and trigger guard. My race gun has a Alchin one sided mount that the frame of the gun was drilled for.
The problem is that the mount has to fit the gun and I don't think anyone will make one for the M&P as it is a production type gun.
If you want a red dot sight, look into a Dr optic or JP optics, they make a small red dot that will fit in the rear dovetail, but its not something you switch back and forth easily.
If you want a fun gun that you can easily mount a red dot to, get a Ruger Mark III with a top rail.