To get the shotgun will require having the keycard for the Security Office which you will be able to find in I.T. Security on the first floor. In I.T. Security grab the keycard on the left and return to the ground of the Lobby and go into the Security Office which is located next to the stairs. There you will find the shotgun sitting on the table along with a book which will have a hint for the safe where you can find the fabrication plan for the shotgun shells.