http://www.rockislandauction.com/viewitem/aid/67/lid/296

One of the options for having multiple shots available in the age of the muzzleloading rifle was the swivel-breech rifle. Such a rifle would have typically two barrels and one lock - one the first barrel was fired, the whole barrel assembly could be rotated 180 degrees to bring the other barrel into alignment with the lock. These could be two rifle barrels, two shotgun barrels, or one of each.

MJ Whitmore of Potsdam, New York build many such swivel-breech rifles and also build some much less common 4-barrel swivel guns. This particular example has two .40 caliber rifled barrels on one side and a combination of a .40 caliber rifle and a .40 smoothbore barrel on the other side. In a particularly neat touch, it holds a single loading rod hidden in a spring-capped chamber in the center of the barrel cluster.