Good point with the barrel, truth is probably somewhere between the barrel and a can. After all, there's also pönttö, both vessan- and linnun-, which again are on the smaller side. I kinda love how much nuance and double meanings there are in especially the spoken language.
True meaning can only be grasped with enough exposure.
Didn't we also have some Vickers six-tonners at the start? Next to useless by then but probably could penetrate at least a BT-5.
Edit: now that I think of it, a canister probably fits quite well between the sizes of can and barrel. But to me it would then be a closed container, which a pönttö doesn't necessarily have to be.