This 12 gauge revolving is very impressive. Other than the barrel, chambers (made from old barrels) and the hammer (from a 1857 Remington revolver), this gun is completely homemade. It cocks using a lever underneath the action.
More photos here.
Russian manufacture MTS makes a commercial revolver shotgun called the MTs-255 Tactical Shotgun.
[ Many thanks to Ztryfe , Christian, Robert and DoubleTapper for emailing me the link. ]