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, 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. 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.

Availability
This weapon was not placed in the game and must be obtained through the console.

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 fisher simply fires a large rifle or shotgun into a pond, using the shockwaves to kill everything).