The One With The Embryos - S4-E12
Trivia: Chandler's line of "Donald Trump wants his blue blazer black" was a genuine flub by Matthew Perry. Everyone's reaction to it was so on-brand for their characters that they decided to use it it for the episode.
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.
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