"Ammo for a grenade launcher. The capsule breaks on impact, soaking the surrounding area with acid."
Acid rounds do heavy damage to all enemies, albeit less than flame rounds. Zombies covered in acid will slough off their limbs, which is useful if the player can hit a group of them in the legs. They have an extremely high stun chance - lickers, G1, and G3 can be perpetually stunlocked by acid rounds until they die.
Acid "sticks" a bit more consistently to enemies than flames. Acid rounds will do full damage against lickers on a wall or ceiling, who take greatly reduced damage from flame rounds due to extinguishing the fire when they fall.
Acid rounds can be crafted by mixing Gunpower with High Grade Gunpower (White).