The SpeedBead System is the FastFire 3 with 8 MOA dot, and specially designed SpeedBead mount. It is available for a variety of shotguns. You can also purchase a SpeedBead Mount, without the FastFire 3 optic.

Apply the ease, speed, and accuracy of a red dot sighting system to your favorite shotgun with this unique mounting system.
The SpeedBead mounts a FastFire optic between the stock and receiver of many popular shotguns.
Your line of sight will fall naturally just above the vented rib, allowing for accurate and instinctive aiming.
At 1x magnification, it's designed for both-eyes-open shooting. That's ideal for shooting clays and hunting waterfowl, upland birds, and turkeys.
Mounts low between the stock and receiver.
Line of sight falls just above the vented rib for accurate aiming.
1x magnification allows for both-eyes-open shooting, good for situational awareness and target acquisition
Parallax free, for improved accuracy.
CR-1632 battery included with automatic time-out feature lasts up to 5 years.

The shotgun mounting system is the only red dot sighting solution for shotguns
Mounts low, aligning the red dot low and just above the bore; keeps the center of the optic directly in line with the rib
The target and red dot stay in focus without the shooter lifting his head unnaturally
A bright red 8-MOA dot allows for fast target acquisition and easy aiming
The compact, lightweight design won’t affect the firearm’s balance or handling
Windage and elevation adjustment make optic fine-tuning easy
High-grade optical glass provides excellent brightness and clarity, with lasting durability
Index-matched, Hi-Lume multicoating aids low-light performance and glare elimination, increasing your success rate
Waterproof
Will not fog internally even in cold and rainy weather
Shockproof construction stands up to years of punishing recoil
Covered by Burris Forever Warranty

Fits 12G Remington 870