The spear sling launches javelins, harpoons, spears and short spears.
"A harpoon, javelin, shortspear, spear, or other weapon shot from a spear-sling is treated as a projectile weapon instead of a thrown weapon, increasing its maximum range and allowing it to be used underwater."
I assume it means to say that it treats the weapon as ammunition, not as a projectile weapon:
Does the spear sling treat these objects as ammunition in any other way, like destroying the launched object?
Do they become thrown ammunition making them incompatible with ricochet toss?
Would the ammo-weapon be retrieved by a blinkback belt?
Would a spear-sling enchanted with shadow shooting be able to launch shadow harpoons with shadow ropes attached?
Does the weapon lose melee only enchantments, such as bloodsong, when used in this way?
How would this interact with the vigilante talent "returning weapon"?
However, if you are treating the launched weapon as a projectile weapon, what the heck does that mean?
However, if you are treating the launched weapon as a projectile weapon, what the heck does that mean?
Three things: 1) increased range; thrown weapons have a max of 5 range increments, projectile weapons have max of 10 range increments. 2) It can be used underwater; thrown weapons can't be used underwater, projectile weapons can (but suffers a -2 penalty per 5 feet of water). 3) Strength added to the weapon damage is limited by the strength rating of the spear-sling.
Spears, etc, shot with a spear-sling do not count as ammunition. That said, if you use a magic spear with a magic spear sling, combine the effects as you would a magic bow and magic arrow.
Ricochet toss is pretty clear about not working with ammunition.
A blinkback belt wouldn't work; you didn't throw the weapon.
I don't think shadowshooting ammo lasts past the hit. No duration is specified but it's pretty magical, lasting forever seems wrong. OTOH it's based on shadow weapon which has a duration of 1 minute/level, you could make a case for that.
Melee only enchantments don't work when not used in melee. In particular you're not wielding the bloodsong spear at any point when using a spear-sling.
'Whenever the vigilante uses that type of weapon, it automatically returns as if it had the returning property.' It sounds like that does work.