The Mysterious Stranger's .44 magnum in Fallout 3 is visually identical to the .44 magnum revolver (save for its front sight, which is slightly smaller), but deals an unstoppable 9000 damage per shot, making it the most powerful weapon in the game, excluding cut weapons.
It has no Pip-Boy icon. Within the Pip-Boy interface, it is referred as ".44 magnum".
It cannot be picked up again once dropped out of the inventory, due to a game engine setting that disallows pickup by the player.
If given to an non-player character (with .44 magnum rounds) through the console, they will use this weapon. However, if used by any non-player character other than the Mysterious Stranger, it will not work properly, doing no damage instead of an instant kill.