I haven't worked out the math or anything, but if you did have a gun muzzle perpendicular to and close to the surface of liquid in a barrel -- water, I assume, in this case -- the blast from the firing could cause any fish in the barrel to float to the surface, concussed and unconcious. But you'd need a pretty powerful gun, perhaps a 12 or 10 gauge shotgun, and it would be a good idea NOT to stick the muzzle into the water.

I also haven't tried this, and don't really want to.

Shooting fish in the wild used to be legal in, I believe, Vermont.