from Races of the Wild-
Cost Dmg (S) Dmg (M) Critical Range Inc Weight Type
Skiprock 3 gp 1d4 1d6 X2 15 ft. ¼ lb Bludgeoning
War sling 5 gp 1d6 1d8 X4 50 ft. 1 lb. Bludgeoning
Skiprock: Halfling weaponsmiths developed these polished stones. Each skiprock is perfectly weighted and shaped for throwing. If the skiprock hits its target, it ricochets toward another target of the thrower's choice. The second target must be adjacent to the original target (no more than 5 feet away). The thrower immediately makes a second attack roll for the skiprock against the new target, with an attack bonus 2 lower than that of the initial attack.
Although they are thrown weapons, skiprocks are treated as ammunition for the purposes of drawing them, crafting masterwork or otherwise special versions of them, and what happens to them after they are thrown.
Skiprocks can be used as sling bullets, but using a skiprock's ricochet ability in conjunction with a sling requires taking the Exotic Weapon Proficiency (war sling) feat.
War Sling: This exotic weapon is a heavy sling used with a special sidearm throw to hurl a skiprock (see above) with deadly power. Without skiprock ammunition, a war sling is treated as a normal sling and deals the appropriate damage when used to throw normal sling bullets in stones.
A user proficient with both the skiprock and the war sling can ricochet a sling thrown skiprock just like a hand-thrown skiprock.
I think it originally appeared in Races of Faerun.