These bullets are actually made from a porous stone or heavy wood that absorbs liquids and shatters on impact.
Soakstones are often used to deliver poison or burning oil. A flask of oil completely saturates 10 soakstones. When ignited (a free action if fire is available), a burning, oil-soaked soakstone deals +1d2 fire damage and can ignite combustible materials (such as a thatched roof ). Allowing a soakstone to burn in a sling’s cup for more than 1d3 rounds gives the sling the broken condition.
Pathfinder Player Companion: Halflings of Golarion. Copyright 2010, Paizo Publishing, LLC; Authors: Hal MacLean and Amber E. Scott.