You must do a quest in order to obtain this gun. To start the quest head over to "Hubris Comics" near diamond city and make your way to the 3rd floor aka the studio. Once at the top you will see a stage and a backdrop, behind the backdrop you will see a replica (fake) silver submachine gun. Afterwards find the silver shroud suit and hat, you should have seen it first but just in case you did not its on the mannequin in front of the stage. Once leaving you will get a silver shroud radio broadcast, tune in and do the quest. The full outfit and gun will be rewards for completing the quest (:

Here is where to find Hubris Comic shop, its hard to see so the image is zoomed in but its just to the right of diamond city!
[ Register or Signin to view external links. ]