Actually it would be the aim of decent gunners and those who know their stuff - that the canister is not bursting inside the barrel. It is evident that for those reasons the iron bottom piece is used, by this then finally - educated guess, shared by Demian - all the projectiles got the identical acceleration and power.
As form my amateur point of view, and reading, it was a problem that a lot of those projectiles had spent their power when hitting the target.
I think some nations tried to counterbalance this by the construction of the canister, as the Austrians for example.
Of course I don't have supersonic eyes - and can also only guess that the idea was to prevent the rupture of the canister inside the barrel (which makes a lot of sense, at least to me)
The ideal canister would burst when hitting the target.