In the A War to End All Wars chapter (World War I, in the church), select either the revolver or the rifle as your weapon. Go into any room that has rats running around. Face towards the rats and press the R (aim). If one of the rats will glows white, you can shoot and kill it with your gun while targeting it. This trick works best when you have complete the game three times and can select "Eternal Mode" so that you have unlimited bullets.