The way I unlocked this was by using the blank bullet on the gun. I waited outside the room with the safe. After about 1:30 Mr. Sixpence will be done looking through the safe and you will be able to pick up the blank round off the floor. Then just quickly bring it to the room with the gun. Make sure you have the bullet equipped and interact with the gun. When the killer tries to fire the gun he will pause, allowing for Mr. Sixpence to defend himself.