Fallout 3 Easter Egg - Dirty Harry

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 the door, 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.

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"!