| Bashe |
Just so you know, Rapid Shot works with crossbows, because crossbows are still ranged weapons, when you have rapid reload. It also works with throwing weapons, also considered ranged weapons, if you have Quick Draw.
I always assumed, that the sling being classified as a throwing weapon - the same way it would take a move action to draw a new dagger to throw but Quick Draw allows full attacks - Quick Draw would allow for drawing and loading bullets at the full attack speed.
Rapid reload is a thing for crossbows also because crossbows actually require certain movements to reload, so i wouldn't consider that applicable to slings. Slings on the other hand would mainly require quick hands and coordination - Quick Draw.
You could also rapid shot a sling. Just as you can rapid shot throwing daggers or shurikens.
Part of the cost difference although it was ignored here is also the small details.
Even if the bow and sling come out to similar damage a crit with the bow is still significantly better and has double the range increment.
Thats what makes it martial.