Canister is an anti-personnel round. It is constructed of iron balls, sometimes (depending on the country of origin) in two sizes, both of iron balls so that they do not melt when fired. Experiments by the French (which can be found in DeScheel's artillery treatise based on Gribeauval's work) with lead musket balls failed because the lead would melt and the desired dispersal effect of the balls was not achieved. The canister body is construced of tin so that the round would rupture in the gun tube upon firing. The base and top of the canister were or iron.
The firing of the piece is what ruptured the canister in the gun tube and the balls did, in fact, bounce along the inside of the gun tube on the way out. The firing of canister gave the piece the effect of a large shotgun and the dispersal of the balls was in a cone shape starting at the muzzle of the piece. A richochet effect for the individual balls of the canister could be achieved on hard ground. Cansiter could also be double-shotted (two rounds and only one powder charge) for close-in emergencies. The canister did not have any internal firing mechanism or fuse. The canister was ruptured by the effect of the charge being ignited inside the gun tube.
This process is outlined quite well using three references:
-DeScheel's Treatise on Artillery translated by Jonathan Williams and edited by Donald E. Graves. DeScheel published his treatise in 1777 and it was translated for US use in 1800.
-A Course of Instruction of Ordnance and Gunnery by JG Benton, published in 1862.
-The Artillerist's Manual by John Gibbon published in 1863.
If you would like specific passages from these volumes as they relate to canister and its functioning, let me know and I'll post them.