You can hurl sling bullets in a plunging arc that increases their momentum as they fall.
Prerequisite: Proficiency with the sling (or halfling sling-staff).
Benefit: When making a sling attack from higher ground, you deal an extra 1d6 points of damage for every 10 feet of vertical distance between you and your target. This extra damage is not multiplied on a critical hit.
*This feat benefits users of the sling or halfling staff-sling, and is the original concept of Weirdo and Xunal on the Homebrew thread of the Paizo RPG website. (Weirdo; Plunging Fire feat)