The first thing you need to find is the Scorpio Key, which is hidden in the piece of meat. Go to the basement and find him. After that, go up the stairs and go to the break room. Once inside, find a broken shotgun there.
For now, all you have to do is pick up the broken shotgun and go downstairs. After you return to the main hall, you should find a statue holding a shotgun. You just need to swap your broken weapon for the one on the statue. This will allow you to open the door directly behind you. Just leave the room and the weapon is yours.
