Successfully complete the game on the Nightmare difficulty to unlock the burst pistol for use in New Game+ mode. Alternately, complete the game with all map fragments found. Then, start a New Game+ mode game with the cleared game data. After you can enter the back area, go to the right where the map fragment holder is found. It will be glowing red if all fragments were collected. Interact with this map, and an attaché case will appear on the nearby table, which contains the burst pistol.