A master-crafted weapon follows the normal rules for the weapon, except that you may re-roll one failed roll to hit per turn. Master-crafted weapons are taken as an upgrade for a weapon that is already being carried by a model and should be represented by a suitably ornate weapon on the model itself. Note that you may not master-craft grenades! The cost listed in the Wargear section is in addition to the cost of the weapon itself (eg a master-crafted power weapon costs 15+15=30 pts). However, only the upgrade costs are taken against the 100 points limit on wargear for a model (so the master-crafted power weapon above would count as 15 points against the 100 points limit, not 30 points).
Master-Crafted Weapons
Source: Codex: Daemon Hunters, p. 17