Explanation: In MythBusters season five, Adam Savage and Jamie Hyneman proved that shooting fish in a barrel is a whole lot easier than finding the original source of that adage. Even the author of the American Heritage Dictionary of Idioms — a veritable prince of proverbs — didn't know who or where it came from.
But despite the mysterious source, the saying is spot on. In fact, Jamie and Adam demonstrated that you don't even have to be a good shot to take out a barrel of fish with single bullet.
When Jamie fired a shotgun into a barrel stocked with 30 plastic fish suspended at varying depths, he struck only three of them, for a 10 percent hit rate. That stat may not be impressive, but the after effects of the shotgun shell in the water will compensate for any missed targets.
Fish are extremely sensitive to the slightest water pressure change thanks to a specialized organ they have called the lateral line, which detects water displacement, force and direction. When a bullet moving faster than the speed of sound strikes the water, it forms a high-pressure acoustic shockwave in front of it. The MythBusters calculated that a 9-millimeter gunshot delivers around 100 g-force units of pressure into the barrel.
Similar to how a loud noise can injure a person's ear drum, such an intense pressure fluctuation from the ballistic shockwave would rupture the fishes' blood vessels and mortally wound them, proving that you don't even have to shoot a single fish to kill a barrel full of them.