Callahan's Magnum

 is a weapon which was added to Fallout 3 with the Broken Steel add-on.

Characteristics

 * Callahan's Magnum is a unique Scoped .44 Magnum that surpasses the Blackhawk (which is the other unique Scoped .44 Magnum) in damage, damage per second, and AP Cost.
 * Depending on equipment and perks, the weapon can fire 1-2 more shots in V.A.T.S. than the Blackhawk.

Location
It can be found in a hard safe within the ruins of the Citadel if you choose to destroy it while onboard the Enclave Mobile Base Crawler during the Who Dares Wins quest. Enter the ruins and find the door that leads to the Armory. The magnum is inside.
 * To be exact, the magnum can be found like this: Enter the Citadel, jump into the very large crater, find the only door in the crater, enter walk up the stairs, take a left until you reach the wall, take another left and you will find a Hard safe and a Hard Terminal to unlock the safe and cubical. The safe is called Callahan's Safe.
 * This is the only upside to destroying the Citadel. Be warned that all Brotherhood of Steel members will be hostile to the player thereafter

Trivia

 * The weapon's name is a reference to Harry Callahan from the Dirty Harry movies who carries a Smith & Wesson Model 29 .44 Magnum as his signature weapon, even though in the films it was never scoped.