In Chapter 5-2, "Uroboros Research Facility", after the conveyor belt area and starting a new checkpoint, you reach a point where you drop down. There are a few crates with flame rounds, rifle rounds, handgun ammo, and an incendiary grenade. Collect everything, then save the game and quit. When you start again, remove the collected items from the item management screen and collect them all again. You can get 300 handgun bullets in a matter of ten minutes.