I don't get what McGrath is on about either. Why would it be difficult again? Because you'd make a hole in the wooden barrel? So what? The phrase is not "like shooting fish in a barrel without making a hole in the barrel."

The point is, there are several easy images (like Mr. Walton's) that adequately explain why shooting fish in a barrel would be easy. McGrath just dismisses or ignores them all. Similarly, it would be really easy to tke candy from a baby, if you don't care about hurting the baby. Not that I'm cruel, just that I can easily see the logic in this expression.

And what has this got to do with a "barrel of monkeys"? In the latter case, it's the great number of monkeys that makes it fun (which I never quite understood, by the way). So it's the fact that it's a whole barrel of monkeys that makes it fun. With fish in a barrel, whether you have one fish in the bottom of an empty barrel, or a barrel chock full of fish, it would be easy to shoot the fish, if you don't worry about making a hole in the barrel. Thus it is shooting fish (one or many), not shooting the barrel of fish, that is easy. But it is the quantity of monkeys (a barrel of them) that is fun.

Got it, people? Good!