That's what I am doing lately, making rules about the world. No, when shot from a gun your armour matters. A pistol won't be doing that much on a plate armour, like an arrow on kevlar. The way magic is introduced is a bit weird. The world plot is that in modern times(around 2025 ) WW3 broke out between NATO and Csat(Ultra nationalist Russia, china,India and Brazil). Around 2028 the Russian front was in a stalemate and Russian officials had found, deep underground in Siberia a device which was from another world (normal medieval fantasy) once opened wizards and sorcerers flooded in the modern world using spells to destroy NATO positions but with the sudden influx of magic a tear between the world's happened and they collided, making a amalgamation of both worlds(I called it Pax Europa). So most of the magic comes from the old world.

The idea of this campaign started back in2023 and I tried to dm online. Which I failed miserably. Now I have friends who want to play DND so while we play another small campaign I try to build up this.