Guys i have a sling in my pocket right now. It is a zero slot item.
Simply put, it functions as the ranged equivalent to a dagger, in that it is inexpensive yet effective in skilled hands.
A blurry string attached to the hand in the throwing animation is your sling.
A sling ignores the ranged penalty from damp weather.
Regular rocks should be usable ammunition, as well as a second alternate led "bullet" with higher damage, range, and weight.
Ammo can be more numerous than bows, but not necessarily lighter; as a kinetic energy weapon, projectiles are generally dense (rocks, clay, lead). Carrying a bagful of lead is heavy.
PS holding the sling consists of crossbow pose minus crossbow with the projectile and pouch in the left hand, attached to the right hand via thongs.