Question: Regarding the scene where people catch fire after John Wick shoots them with a particular gun, what exactly is causing them to catch fire? I recall an earlier scene showed men carving lines into the tips of their bullets, though I don't remember if the two things are related. If so, is there something about carving the bullet that causes it, or is it the particular gun?
Answer: It may be incendiary ammunition (not to be confused with tracer bullets). The bullets have a hollow head containing a flammable mixture that ignites upon being fired.
Answer: These are dragon's breath shotgun shells. The shells are filled with magnesium pellets and ignite when fired. The film seems to exaggerate the stopping power, making it seem almost like an explosive round rather than incendiary. These are not the same rounds that we see the men carving into. Carving grooves into a bullet is done so the bullet expands upon impact and causes more damage similar to a hollow-point round, though the effectiveness of this method is debated.