Are you sure he didn't say convertible between semiauto and pump-action? 'Cause the're two shotguns like that, the Benelli M3 Convertible and the Franchi SPAS-12 (actually there's another one, the SPAS-15, but that doesn't look so conventional.)
Anyway, the only fullauto shotguns I know of that are supposed to be fullauto are the USAS-12, Pancor Jackhammer, and the H&K CAWS. I know plans for converting the Remington 1100 to select-fire used to be available, but that would be not only illegal, but impractical and painful.