Pa's Fishing Aid

 is a unique version of double-barrel shotgun added to Fallout 3 with the Point Lookout add-on.

It looks identical to its regular version, the double-barrel shotgun, but differs in damage, price and other characteristics. It can only be repaired with combat shotguns.

It deals less damage per shot than the double-barrel shotgun but this is made up for with a higher Critical Hit damage and a much lower spread. Pa's fishing aid, without any perks, in a single Sneak Attack Critical to the head outside of V.A.T.S. deals a whopping 3200 points of damage, which would make it powerful enough to kill just about any enemy in the game in one shot.

Location
Located behind the wardrobe to the right of Marguerite, in her shack.

Behind the scenes
The name is most likely a reference to the commonly known idiom "shooting fish in a barrel" or a method of fishing known as "shotgun fishing" - in which a fisherman simply fires a large rifle or shotgun into a pond, using the shockwaves to kill fish.