In trying to have magical adventures in the modern day one must find a way to explain why heroes don’t use guns. After all, if facing a demon would you rather have a sword or an AK-47? Some games handle this by making magical creatures have what amounts to tank armor for skin. An interesting, if not completely convincing idea outside of certain settings.
Of course, the easiest method is to claim magical creatures are invulnerable to modern weapons. To my mind, and many others, that is a cop out.
That said, if I want A Demon Haunted World to be modern sword and sorcery adventures I need a way to limit guns, flamethrowers, and artillery, among other things. To do this I’m going to look at modern weaponry a little differently. What if the wasn’t from the bullet or the flame but from the source: a strange mixture that propels the sling stone instead of a sling or a strange mixture that burns instead of lamp oil?
Well, in the S&S world, especially in the context of Old School fantasy gaming we have a word for strange mixtures that can harm you: potions. We also have a way to escape harm from potions: saving throws. Why not allow creatures of myth and magic (including my Blooded race of humans) a saving throw against potions when attacked with modern (gunpowder and later) weapons, flame, and explosives. A successful save resists the potions effects and converts the damage into the equivalent muscle powered weapon: sling stones, natural fire, and so on. Or perhaps a save allows no damage. Tougher (more magical) creatures might get a plus on this save to reflect their distance from the modern world.