This interesting thing called "refraction angle" tends to make it much less than straightforward.
Except in the widely used expression semantics, the 'shooting' is actually done with a rod and a line and a baited hook, not a firearm or bow. 'Shooting a line' is the expression used to describe casting the lure in fly fishing.
In a similar vein, the expression 'shooting someone a line' means trying to deceive them with a decoy - like an angler does with a fish - And this is also what berman has been doing with DECN, it so happens.
Therefore 'shooting fish in a barrel' could actually be done quite succesfully and with minimal effort by someone even with their eyes closed. So 'refraction angle' does not enter into it.
'Fly-fishing' a fish confined to a barrel would be so easy as to be 'unsporting'. And this is the derivation and meaning of the idiom 'shooting fish in a barrel'.