If you want the most effective way to mount a red dot to your shotgun while having propper cheek weld take a look at the AR15 buttstock adapters we manufacture for the remington 870, and coming soon the Mossberg 590/500
I designed the adapters for the primary purpose of being able to mount optics effectively and have the propper cheek weld to use them for competitive shooting, hunting, military and law enforcement applications.
The rail comes with each adapter and attaches via hex head screws. The rail mirrors the height of an AR15 flat top upper receiver identically so your cheek weld between rifle and shotgun are identical for easier cross training.
The adapter also uses any AR15 buttstock and any AR15 pistol grip.
I just took delivery of the first prototype Mossberg 590 unit this week, and it looks excellent, I hope to have a production run done before or after SHOT Show (2nd week of february)