You can find a unique Gatling laser weapon called Vengeance under an outcropping of rock jutting above a bloody lake at the far rear of the Deathclaw Sanctuary. Go as far southeast into the sanctuary as you can. It should open up into a large cave. You will see a large rock in the center of the cave. It is approachable from the south, and is illuminated by some glowing fungus. The bloody pond is filled with corpses, and next to one of them is Vengeance, which is in relatively good condition when found.