in the room where you choose the starting weapon is a computer that will give you codes to input, one of them is a free pap gun. after you enter the code on round 3 there will be a vase that spawns in the main area of the map with lightning striking at it. this is a soul box taking around 5 zombies to fill on round 3. after the first one is filled another will spawn and repeat the process until you get the onscreen prompt telling you that a free pap gun is near Jug