Mag fed shotguns have been around for decades, but they were hard to find. Not anymore. Now you can upgrade your pump-action shotgun, find a suitably AR style shotgun, or take a hint from Owen Zastava Pitt and run an AK style semi-automatic shotgun.
The true AR pattern shotgun is still elusive, but Rock Island Armory makes a solid 12 gauge stand-in. Check out the VR-80.