The Desert Eagle serves as an excellent mid-game pistol, offering higher damage than the old 10mm, and a fairly common ammunition type. It out-ranges the .44 magnum revolver by a bit, and carries 2 more rounds in a magazine, but the .44 magnum does more damage and can be fired for fewer AP. Overall, this weapon is great until you can get your hands on a needler pistol or .223 pistol. Could be replaced by the 14mm Pistol, but only with an armor-piercing fix patch.
Can be obtained by looting the corpse of officer Jack in NCR, during "Take care of Officer Jack" for Mira. You can either kill him, or let him blow himself up. It's in the same building where you see Dorothy and Toto the robot.