To start this quest you must travel to "Good Neighbor" after arriving here you will see a small alley on the left hand side, walk down it and make the first right and follow the path and you will see a door, approach the door and you will enter a conversation. Speak with Bobbi No Nose" Do all her dialogue and do what she asks and you will star the quest called "The Big Dig", im not going to run through the quest because thats your job but at the end of the Quest when you meet "Fahrenheit" you will have a few options, but either way you get the gun but I would recommend persuading "Bobbi No Nose" to leave aka the peaceful route. You can either convince "Bobbi" to leave , kill Fahrenheit and all the raiders, or kill Bobbi.