I've never read the story, but it sounds like the man on the bluff is shooting at a fish with a gun. (A bluff is a cliff or a hill.)
Since I worked on the sights of that gun the other day Pa couldn't hit a hog with it
The "sights" on a gun are the part that you look through in order to line up your target before shooting. If the sight is bent or out of alignment, the shooter will miss his target. This child did something to the sights of his father's rifle so that he wouldn't be able to hit a large target like a hog, much less a fish.
Student or Learner