This idiom has always confused me. I’m no expert with a gun—I’ve only shot one once at my then-boyfriend’s house down in Texas—but it doesn’t seem like shooting fish in a barrel would be the easiest thing.
First of all, we usually catch fish, not shoot them. It seems to me that is for a reason. Maybe it’s because fish move around so much and so constantly. Maybe it’s because water can mess with your depth perception, which is important when trying to be an accurate shooter. Maybe it’s something else entirely, I’m not sure!
Also, if you’re shooting fish in a barrel, that barrel is going to have holes in it. That will allow the water to drain from those holes, which will mess with your water-to-fish ratio. I suppose that would make it easier though, because as the water drains the fish have less space to swim away to.
After a while you’d be shooting fish in an empty barrel, which would be way easier. And after even more time, you’d essentially be shooting dead fish that were lying at the bottom of that empty barrel.
So I guess this idiom makes some sense. Either way you’re going to end up with a bunch of dead fish—goal achieved!