That is the what I have been struggling to understand. Just some thoughts:
1. The Cannister is propelled down the gun tube.
2. Heating from the deflagration of the gun powder softens the solder.
3. When the cannister leaves the gun tube, it starts to tumble.
4. The lead solder fails at 180-300C causing the bottom, sides and possibly the sides fails in flight distributing the contents.
5. A proportion of the the cannisters could fail upon hitting the ground but this is likely to be small.
Notes upon Gunpowder [Garbett (1897) Nava Gunnery 216-9]
The ignition temperature for reaches 482 F, the sulphur which is present to lower the ignition point bursts into flame causing the potassium nitrate to release oxygen that oxidises the charcoal to form carbon dioxide and carbon monoxide whilst releasing nitrogen. About 43% of the mass produces gas with 57% as solid matter. The temperature reaches about 4000 F. One gramme of powder yields about 0.26 litres of gas. Gunpowder of this period was in the form of pebbles made from press cake cut into cubes that are glazed with graphite. Small grain powder used for muskets or carbines gave a large ignition surface and would be consumed by the time the projectile left its seat so producing huge inital pressures.
It was estimated that about 10-15% of powder was not consumed. Only with the introduction of prismatic gunpowder of six sizes with a hole in the centre that
the efficiency was increased for the introduction of rifles ordnance in the 1860s.