Well, weight doesn't affect the speed of a falling object, but it does affect the amount of force it would fall with. The lowered force would mean the air resistance would have a greater effect, therefore indirectly lowering the speed you'd fall with. 

Plus, I don't think that proposed weapon could be made with the technology level of this world. It would take some incredibly precise machinery.

Also, all that skystone would be rather expensive. Not a problem, as long as you have a reason why your character can spend that much money on weapons.