This system by Roth Concept Innovations adds what are essentially four magazine extension onto your shotgun. Rotate the tube to access the extra extensions:
Though innovative and more rounds are always better than fewer rounds, I wonder if I’d really care for the extra weight up front. The full version weighs just over 2 pounds empty.
Particularly nice is the fact that as rounds are fired, the XRAIL rotates to keep feeding rounds down the main tube and that no permanent modifications to the gun are required.
More info, including videos, at the RCI site.