Runite limbs are required to make a rune crossbow. They can be smithed from a runite bar at Smithing level 91, granting 75 experience.
Runite limbs can be attached to a yew stock at Fletching level 69, granting 100 experience, to make a Runite crossbow (u). They are a common drop from Metal dragons.
Note that the High Alch value of the complete crossbow is only 120 coins more than the limbs alone. It is more economically sound to use the yew log for a yew long and alch it rather than use it on a rune crossbow. The limbs cost around 8,000 coins up to 12,000 coins and can be at times hard to buy or sell.