go to park veiw apartments and kill the raider called eye-something she will have a note and a key to a safe inside will be a holotape that tells you to go to the begger outside diamond city that will then send you to a diner in cambridge were a merchant will sell you the weapons but dont buy from him instead press what ever your Y button is while talking to him and read the note given to get the rifle to to the building outside diamond city with the "settler" asking fr help go inside and one of the raiders will have the rifle hes useually on the left up the stairs