Reload the revolver really fast.

In Act 1 Scene 6 (Checkpoint Charlie), you will come to a memory where you learn how to shoot a gun. Once you get told to shoot a can, don't. Hold the RS to draw your gun and press RT to shoot all 6 bullets. Now, press LB quickly to reload your gun. If you don't do it fast enough, you can keep refilling your bullets with the ammo boxes to your right.