I recently came across Black Aces Tactical, a Florida company making quad-rails for the Remington 870/1100 and Mossberg 500/590 shotguns.
The rail system is a single piece made from 6061-T6 aluminum alloy weighing 16 ounces. The rails are black anodized (type II). The rails will accept any standard AR-type accessories.
Black Aces Tactical states felt recoil will be reduced by about 15%. There is not a specific explanation on the website on why this is, but I imagine the additional weight on the top and end of the gun help reduce the recoil impulse and muzzle rise.
According to Black Aces Tactical, the shotgun rails only take about ten minutes to install. The quad-rail system will fit both tapped and untapped receivers. The installation kit comes with the tools needed for installation.