Mithril bolts are a ranged projectile which are fired from a mithril crossbow or higher (requiring 36 or higher Ranged).
To create mithril bolts, a player must first smith unfinished mithril bolts by using a mithril bar on an anvil while carrying a hammer. This requires a Smithing level of 53, and for each mithril bar that player smiths, they will get 10 unfinished mithril bolts and 50 Smithing experience. Then, a player must attach feathers to the bolts. Attaching feathers to each bolt requires a Fletching level of 54 and 1 feather, granting 5 Fletching experience per bolt.
Mithril bolts may be tipped with sapphire, emerald, or a grapple. To tip the bolts with sapphire, a player must have a Fletching level of 56 and use sapphire bolt tips with the mithril bolts, granting 4.7 Fletching experience for each sapphire bolt tip attached and making the mithril bolts into sapphire bolts. To tip the bolts with emerald, a player must have a Fletching level of 58 and use emerald bolt tips with the mithril bolts, granting 5.5 Fletching experience for each emerald bolt tip attached and making the mithril bolts into Emerald bolts. To tip a bolt with a grapple, a player must use a mithril grapple tip on the mithril bolt, granting 11 Fletching experience, and also a rope, making a mithril grapple.
They are commonly used for Ranged training due to them being modestly priced with a good damage output.
For an exhaustive list of all known sources for this item, see here