The Ratslayer is the Fallout New Vegas rare / unique version of the varmint rifle, and is one of the least powerful and sexy rare weapons in the game. It's a damn sight better than a normal varmint rifle though, and this video will show you how to find it.
From the Creator:
The Ratslayer can be found next to a desk inside Broc Flower Cave. sometimes the Ratslayer would have fallen off the desk and will be on the floor.