Blood

Simple. Get vanilla blood particles and decals, then use them to my advantage. Making the blood decals small, particles turn into droplets, and droplets turn into little blood drops. Eventually, after certain amount of shots, the NPC (or player) will bleed out and die of blood loss. Blood can splatter on walls, ceilings and floors. Only spawning on ceilings if the NPC is tall enough, or if you're in a situation where you're really close to the ceiling.

Weapon to blood physics

The weapon does raycast things and makes funny bullets. Well, what I did was catch that raycast and bullet thing and use it to see where to send the blood to. Easy as that.

This mod generates its own blood decals, droplets, and puddles at runtime, so no prefab setup is required. Don't worry about SDK mods or something like that, since it's already done.