It's called a duck's foot and all barrels fired at once. I've seen originals with as many as eight barrels. It was indead a naval weapon and was used for boarding other vessals. It's was also used as defence from mutiniers.
Bill
I just watched the R Lee Ermy show "Lock and Load"... the Pistol episode...pretty good show; he shot one of these... strange to see one twice in 2 days!