In talking about french cannister:
The iron plate at the bottom of the tin cannister was to 'prevent the charge from going through the cannister' and it rupturing prematurely. Scheel Otto (1795) Memoires d'artillerie; contentant l'artillerie nouvelle our les changements faits dans l'artillerie Francasie en 1765' 2nd Ed. see also Allix 'System d'artillerie de campagne du General Alix'.
Allix also points out that at least half of the balls from a cannister round had no effect and were lost to the ground, implying they did not ricochet.
Under the AnXI reforms, one type of cannister was developed. It had a mix of large and small balls, with an iron base plate as it afforded greater effect upon the balls. In addition, if the cannister ruptured prematurly the bore became worn or damaged. The top of the cannister was no longer soldered on, but held in place by crimping over the cannisters top edges. Ergo the top was more losse fitting than previously.SHAT 2W 84 also 4w 84
All four sources talk about preventing the cannister from 'opening' prematurely. I have no idea what the normal 'opening' was intended to be. The inferance being if the cannister opened too early the gun tube could be damaged by preventing this premature opening.