Callahan's Magnum

 is a weapon which was added to Fallout 3 with the Broken Steel downloadable content pack.

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 (Spoilers)
The magnum is found in a safe within the ruins of the Citadel if you choose to destroy it while onboard the Enclave mobile base during the Who Dares Wins quest. If the Citadel is destroyed enter the remnants and in a hidden enclosed office is the Magnum.

Trivia

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