Every body knows the explosive bullets made by the french gunsmith Devisme.
The are many variations (brass tube, steel tube and so on)
Most of them were designed for big game hunting (elephants, tigers aso) rifles and the diameter is around 18 mm.
Devisme made also guns for whales hunting with the same kind of explosive bullets.
They are a lot scarcer.
tube diameter 30 mm
total length: 110 mm
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He designed also harpoon guns for whales;
The bullets not only have a hook for the line but are also designed to explose !
The gun has a groove in the barrel because of the hook
Here are some pictures of whales projectiles and the whale harpoon gun
(The explosive bullet is not for this gun but for a whale gun)