Fish in a Barrel

"Like shooting fish in a barrel." It's a common phrase used to express utter lack of a challenge in doing something that is probably unsportsmanlike. But as a metaphor, it seems like a rather strange one. I've never heard of anyone shooting fish in a barrel nor can I imagine a purpose in it. That being said, the scientist in me is extremely tempted to conduct an experiment to determine the result of such an undertaking so as to determine both (a) the ease with which it is accomplished and (b) the result and, hopefully, hence any reason for doing so. That being said, if you see me with a barrel, a net, and a gun, please stop me.