The shotgun is a powerful weapon that can be found in the main hall of the the building where you meet family Baker. First, get the Scorpion Key. It is found in the cellar with the slime monsters. The key is sticking out of a piece of meat. Next, use the Scorpion Key on the recreation room upstairs (go up the stairs from the main hall) to find the broken shotgun inside. Then, go back downstairs to the main hall and take the shotgun from the statue and replace it with the broken shotgun to make the door behind you open so you can escape. You can then repair the broken shotgun. After escaping to the yard, look under the porch to find a removable metal part. Behind it is a box that contains a repair kit. Combine it with the broken shotgun to get a working shotgun.