The USAS-12 is one of the few fully-automatic shotgun to actually be put into mass production, aside from Russian 12ga AK conversions. It was designed in the late 1980s based on the work of Max Atchisson and manufactured by the Daewoo conglomerate in South Korea. About 30,000 were made in all, mostly sold in Asia for police and military use. Importation of semiautomatic examples into the US was hindered by the Treasury Department determining them to have no sporting purpose, and thus to be legally classified as Destructive Devices under the NFA.
Edited to add that a commenter on Youtube pointed me to a copy of the original marketing film for the USAS-12. You can see it here: