Callahan's magnum is a unique scoped .44 magnum that surpasses the Blackhawk (another unique scoped .44 magnum) in damage, damage per second, and AP Cost. It's the most powerful Small Guns handgun in the game used by the player, surpassed only by the Mysterious Stranger's .44 magnum (which is unavailable to the player without console commands).
It can be found in a hard safe within the ruins of the Citadel if you choose to destroy it while on-board the Enclavemobile base crawler during the quest Who Dares Wins. Enter the door at the bottom of the crater, head up the stairs, and enter the cubicle against the far wall. The safe requires a Lockpickskill of 75, but there's also a terminal you can hack with a Science skill of 75.
This weapon is unobtainable if you choose not to destroy the Citadel, other than the use of console commands.
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. Indeed, if you use Callahan's terminal to unlock the safe, it is revealed that this certain Callahan's first name was "Harold".