The material below is Open Game Content from a 3PP Source
Juggle Load (Combat)
Your fingers are so nimble that reloading your sling is almost effortless for you.
Prerequisites: Ammo Drop, Sleight of Hand 1 rank, proficient with sling.
Benefit: You can load a sling or double sling as a free action. This does not provoke attacks of opportunity. This feat allows you to fire your sling as many times in a full attack action as you could attack if you were using a bow.