"Ammo for a grenade launcher. The capsule breaks on impact, soaking the surrounding area with acid."
Acid Rounds are the standard ammunition used for grenade launcher the GM 79. In addition, they cannot be found at various locations in the game, but can only be crafted by combining one Gunpowder with one High-Grade Gunpowder (White) or one Gunpowder (Large) and one High-Grade Gunpowder (White).
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.