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The sling hurls lead bullets. It's not as easy to use as the crossbow nor as powerful as a bow, but it's cheap, and easy to improvise from common materials. Druids and halflings favor slings.
A character can hurl ordinary stones with a sling, but stones are not as dense or as round as bullets, so they deal only 1d3 points of damage and suffer a -1 penalty on attack rolls.