Another option;
http ://www.or3gun.com/OR3GUN/about_or3gun_sightsaddle.htm
Sight Saddle
Optic
(FF3)
Gun Sports
Hunting
Requires Micro
Red-Dot
(Vortex/Burris)
Includes 8-40 Screws
(Receiver Mount)
The Sight Saddle - Optic is a sleek way to mount a micro red-dot from Vortex (Venom/Viper), Burris (Fast Fire Series) or Trijicon (RMR/SRO) to your Mossberg shotgun. As most Mossberg 500/590 and 930/935/940 series shotguns manufactured since the mid 1990s feature an 8-40 tapped receiver, the Sight Saddle Optic is the lowest profile, most efficient approach to mounting these optics in a position that gives a wide field of view and requires the least amount of rise adjustment to your stock. While there are receiver to stock optic mounts for other shotguns, the Mossberg's tang safety has left most companies to ignore the Mossberg when it comes to optic mounts. Until now, the only official option has been a receiver rail and factory rail mount. These are considerably higher in profile and compound the cost for guns without an existing rail.
Contact for international orders to see if mounts qualified for shipping.
http ://www.logiztix.com/e-mail-o3g.htm