There may be Mutant Zombies that are immune to your shotgun pellets or 9mm bullets. Do not fret, as here are a few ways you can combat these types of zombies.

- You can throw grenades at them from a safe distance. Throwing grenades at point blank range is dangerous as you will be engulfed inside the explosion followed after grenade detonation.
- You can Melee combat them from their sides or backs. You can use the Red Flag to deal efficient damage upon them.

You can also stall them using metal walls and prioritize zombies that aren't bullet resistant to get rid of any distractions while in combat with the bullet immune zombies.