The rules here are more accurate representation of ancient slings.   They were harder to use but did similar damage and better range than a long bow.  The PHB slings are closer to a sling shot than a real weapon.

Greek Sling

Martial Weapon, ranged weapon

1 gp, 1d6 bludgeoning  - ammunition 200 ft/700 ft range  (Difficult)

Can be used with normal rocks, regular sling bullets, or Lead Greek Sling Bullets.  When used with Lead Greek Sling Bullets, damage is upgraded to 1d8.  Adamantine versions also exist, also doing 1d8, 20 cost  400 gp

Difficult weapons have -5 to hit, and on a natural 1 they do damage to a randomly chosen person within 5 ft of the user. (natural 2 if you already use a critical failure system for all weapons).  If no one is near the Slinger, they merely miss their shot.   Difficult weapons can not be fired prone.

Lead Greek Sling Bullets (20)

Ammunition

1 sp, 2 lb

These bullets upgrade damage only with a Greek Sling, doing 1d8.   Adamantine versions exist that cost 400 gp for 20 adamantine bullets

Greek Sling Expert

Prerequisite (Proficiency with the Greek Sling)

• Increase your Dexterity by 1, to a maximum of 20.
• You no longer take the -5 penalty Difficult to hit for using a Greek Sling
• You can stow another weapon the same round you draw a Greek Sling, or stow a Greek round the same round you draw another weapon.