Joe, regarding ML doubles whose barrels have let go, some years ago I had the opportunity to "guest curate" the several hundred firearms in the Southern Oregon Historical Society's collection. There were a couple of dozen muzzleloading doubles among them, and three or four had burst barrels -- always the left barrel and typically near the middle of the forend. My thought was that these must have been related to the right barrel being loaded over and over and the left not discharged until the components had moved. Just a theory, but I bet it cost the shooters their thumbs.