The material below is Open Game Content from a 3PP Source
Ammo Drop (Combat)
Your coordination is so perfect that you can simply drop ammunition from your hand at the exact moment required for it to fall into an open sling as your twirl it around.
Prerequisites: Sleight of Hand 1 rank, proficient with sling.
Benefit: You can load a sling or one end of a double sling with one hand as a swift action or move action. This does not provoke an attack of opportunity.