I suppose a simpler (if corner-cutting) solution would be to rename all the arrows as "missile". Making them a "missile of fire", a "missile of darkness", and so on, with a vague icon. It magically becomes an arrow when shot from a bow, a bolt if shot from a crossbow, or a bullet if shot from a sling.
Personaly i would hate such solution ... :-/
It would seem just as weird and unimmersive as firing arrows by sling.
