I think a candiate for if not largest shotgun, then large shotgun that has actually been used in service would be the Russian KS-23, at least the original barrels were made from rejected 23mm barrels for anti-aircraft guns, is a 4-gauge by European measurements, a 6-gauge by US standards. And techincally it's a "carbine" because the barrel is rifled (though the civilian version is a smoothbore)
Then there's the Nock Gun, several musket barrels welded together as a English Navy as an anti-boarding device, like a portable mitrailleuse. Only a few of the Royal Navy's largest, heaviest, and craziest could fire it without dislocating their shoulder. When fired down from the rigging it tended to set the sails on fire.