I've never thought about this one before, but I wonder whether PoppaGator could be on to something.

The line says fish in a barrel, not in a barrel full of water. This makes me think in terms of barrels of herring, pilchards, or similar. Fish used to be shipped in this fashion, packed in so tight that if you had the means to shoot the barrel at all, you could hardly fail to hit a fish.

I have no grounds to support the theory, it's just an idea.