Flame Weapon and Magic Weapon seem to correctly apply to throwing darts, but not to slings. I'm not entirely certain that flame weapon is meant to apply to slings, though they are technically throwing weapons, but magic weapon I'm sure is meant to apply.
Sorry in advance if this is user error. I'm still stumbling my way through NWN mechanics.
I don't BELIEVE Flame Weapon is supposed to apply to slings - Unlike Darts and Throwing axes, it's the bullets that are being set on fire, not the sling. So it would be treated like Bows and Crossbows.
But... The Magic Weapon not applying is definitely a bug, I believe.
Not a bug unless another DM can correct me and say it should work on them, and I am not aware of it.
But no, those spells do not work on devices like bows or slings.
Magic Weapon works on crossbows and bows, I guess it would make sense for it to work on slings as well.
Straight from the spells changes :
Magic Weapon
Applies to bows, crossbows and all other ranged weaponry. Unlike Flame Weapon.
Well I will have to confirm from other scripters if it was added, and how it was added. If it was, it seems it will only be attack bonus and likely not in item iself as a item property buff, but in character sheet at most. So keep an eye on those attack rolls in combat to compare if you have it or not.
Flame weapon should not work on bows / crossbows / slings. Magic weapon was supposed to be working on them.
Both spells can work on actual throwing weapons (axes, darts, shuriken, etc)
On a side note if someone investigates these scripts - Gloves are able to receive both spells, but the damage bonus only applies of the second spell. (If you cast flame weapon on gloves, it works. If you then cast greater magic weapon, it overrides flame weapon with +2 or +3 damage depending on level). If you cast greater magic weapon first, then flame weapon, you maintain a +2/3 AB (depending on level of caster) and get the flame weapon damage bonus. They should receive both effects (base damage type for magic weapon spells, temp bonus elemental damage of flame weapon)
Magic Weapon should ideally grant +1 AB to slings as it does for other ranged weapons.
So this is a bug.