Bronze bolts are the weakest crossbow bolts, and may be shot from any crossbow. These are the only bolts that are available to free-to-play, usable with the crossbow and phoenix crossbow. Using them in f2p is not recommended.
Bronze bolts may be created by members. First, a player must smith unfinished bronze bolts. This requires a Smithing level of 3, and for each bronze bar that player smiths they will get 10 unfinished bronze bolts and 12 Smithing experience. Then a player must attach flights to the bolts. Attaching flights to each bolt requires a Fletching level of 9 and 1 feather, granting 0.5 Fletching experience per bolt. They may also be stolen from the crossbow stall in Keldagrim.
Bronze bolts may be tipped with Opal or barbs. To tip the bolts with opal, a player must have a Fletching level of 11 and use opal bolt tips with the bronze bolts, granting 1.6 Fletching experience for each opal bolt tip attached and making the bronze bolts into opal bolts. To barb the tips of the bolts, a player must have a Fletching level of 51 and use barbed bolt tips with the bronze bolts, granting 9.5 Fletching experience for each barbed bolt tip attached and making the bronze bolts into barbed bolts.
Interestingly, before the crossbows update, there was only one kind of bolt, called simply "bolts." These bolts could be tipped with barbs, pearls, and opals but not enchanted. With the arrival of the crossbow update all of the regular "bolts" became bronze bolts.