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This location is due west of Vault 63, up the hill at the end of the road. It is a small two-room building with assorted junk inside and a first aid box. There are some makeshift shelters in the front and back of the building, one of which contains a sleeping bag and a couch. There is a weapons workbench behind the main building.
Hornwright testing site 3 appears only in Fallout 76.