Before you can get the shotgun, you need to get the keycard for the Security Office. It is located in I.T. Security on the first floor. You can get into the room with a Level 1 hack. Prepare for a fight and grab the keycard on the left. Return to the ground floor of the Lobby and go into the Security Station, located next to the stairs. The shotgun is sitting on the table. There is a book next to it with a hint for the safe where you can find the fabrication plan for the shotgun shells.