To get this gun you will need to solve a very simple puzzle, luckily the answer is written on the wall behind you. Anyway, this area is NOT a location on the map BUT it is across the river from "Finch Farm", you will see a road that ends in a circle style road. The area is there. After arriving go through the door and follow the path to the bottom of the stairs where you will a machine with 4 buttons. Press the buttons until the code reads 10,4,5,1 , the door will open and the gun is sitting on the floor next to a skeleton.

Here is the location of finch farm and across the river is where you need to be in the cul de sac .
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Here is a video for you guys