Having slings in the game would be really cool. It’s rarely used, so it would makes for something that sets the game apart from others. It would also add another tactical option that’s different from the bows and javelins. The biggest difference with bows would probably be that it’s less effective against armour – especially when it diverts impact or absorbs it – though it can definitely shatter or puncture stuff. The relevant attributes seem to roughly amount to:
-Cheap and easy to make. Cheap ammo – even if you use lead or clay bullets.
-Suffers from rain, but not as much as a bow.
-Can be used one-handed. Is more easily used on the move than a bow. Excellent weapon for a skirmisher.
-Impact damage. Can shatter bones. Causes debilitating internal injuries. Very deadly when it hits the skull. This would be pretty relevant to the
upcoming injury system. Best against lightly armoured or unarmoured opponents.
-Less penetration than arrows, leading to usually being less effective against armour.
-Harder to learn and master. Has it roots in hunting like the bow and not really used in urban areas.
-Not suited to tight formations or tight spaces (fortifications). Can’t fire over allies in a formation. Needs space.
So as a weapon you could have it be less effective against armour than a bow or crossbow, but it’s still deadly when a hit gets through. It’s also cheap, maybe with a special ability to scrounge for somewhat weaker ammo on the battlefield? Being one-handed also means that you can also use a slinger as a skirmishers, while having it use less fatigue and having more ammo than a javelin – though less damaging. It would also be affected somewhat less by poor weather conditions than a bow.
You could have the ‘hard to learn’ and ‘need for space’ aspects reflected by giving it a greater chance at friendly fire when firing into the lines. That way the player can organically overcome the problem by raising the ranged skill.
You can use it pretty well for hit-and-run without giving up the protection of a shield, so it might make for a good weapon for the Goblins. They seem like the type to use skirmishers and ranged weapons, with this one being a nice way to harass a formation of heavily armoured mercenaries to lure them into some traps.