SRM Arms have developed a semi-automatic shotgun that has a 16 round detachable rotating magazine that takes 2 3/4 or 3″ shells.
The overall length is 34″ with 18″ barrel. The NFA version has 12 or 9 inch barrel with a magazine capacity of 12 or 8 rounds.
The only thing that would make this cooler is a bullpup version, although I do not know if that would be possible with the magazine.
More info here.
Hat Tip: DRTV