ATI is now selling an adapter to mount an AR-15-compatible stock on a 12 gauge Mossberg (500, 535, 590, and 835), Remington (870) and Winchester (1200 and 1300) shotguns.
The Talon mounting system, including the pistol grip, has a low MSRP of MSRP of $44.99. With the 6-position stock pictured above it costs $134.99.