Mithril bolts (unf) are made by a player with level 53 Smithing using a mithril bar on an anvil. You get 10 mithril bolts (unf) for each mithril bar and gain 50 Smithing experience. At level 54 Fletching, you can add feathers to make Mithril bolts each bolt will grant 5 Fletching experience. These are a popular choice when it comes to Smithing experience with good experience rates at a fair price.
This is also a popular choice for players who are power-levelling Smithing at places such as the Blast Furnace and is considered to be one of the fastest methods of levelling Smithing in game. Also, you can buy the mithril ore and coal yourself and smith mithril bars using the Blast Furnace. A profit can be made making bolts from the bars and adding feathers and then selling them. Also, you can make them into Emerald bolts and enchant them for even more money.