Benton, James Gilchrist (1862) A Course of Instruction in Ordnance and Gunnery,
Benton is explicit on the barrel rupture and notes (pages 80-81) that "Case-shot are a collection of small projectiles enclosed in a case or envelope. The envelope is intended to be broken in the piece by the shock of the discharge...". Case-shot then being defined as "Grape-shot" (i.e. 9 "larger" cast-iron balls in a metal framework) or "canister-shot" (i.e. 27 "smaller" cast iron balls in a tin cylinder with "...at the bottom a thick cast iron plate and at the top by one of sheet-iron"). The grape is open between the balls, but canister is "...closely packed with sawdust” to "give more solidity to the mass and prevent the balls from crowding upon each other when the piece is fired"
However there is a foot note on page 81 "The balls for canister for bronze rifle-guns are made of lead, or enclosed in a case of some soft material, to avoid injury to the surface of the bore".
In this case the real issue may be the rifling, but then that is quite coarse for guns of this time. I understand that the issue may be about the more repetitive "dinging" of loose canister balls is the issue on brass. Each "ding" causes a slit chip and roughens the barrel. The rougher walls provide more leverage for subsequent canister balls to initiate cracks in the tube. Whereas a ball creates a grazing effect which "sands" rather than "chips" the wall of the tube. Personally I don’t know but anyone with a metallurgy bent is welcome to give a more technical explanation.