Shooting fish in a barrel
Shooting fish in a barrel is an idiom
Idiom is an expression, word, or phrase that has a figurative meaning that is comprehended in regard to a common use of that expression that is separate from the literal meaning or definition of the words of which it is made...

, describing an effortless or simple action, with guaranteed success.

The Mythbusters
MythBusters is a science entertainment TV program created and produced by Beyond Television Productions for the Discovery Channel. The series is screened by numerous international broadcasters, including Discovery Channel Australia, Discovery Channel Latin America, Discovery Channel Canada, Quest...

 team tested and proved the accuracy of the idiom. If the fish does not die from the shot, it is quite likely to die from the shockwave.

Before the days of refrigeration, fish were packed and stored in large barrels. The barrels were packed to the rim full of fish. Any shot that entered the barrel would hit at least one of them. Thus nothing can be easier than shooting fish in a barrel.
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