A master-crafted weapon follows the normal rules, except that you may re-roll one failed To Hit roll per turn for an attack made by the master-crafted weapon. Master-crafted weapons are taken as an upgrade for a weapon already being carried by a model and they should be represented by a suitably ornate weapon on the model itself. Note that you may not master-craft grenades!
The cost of a master-crafted weapon is in addition to the normal cost of the weapon. However, only the actual upgrade will count against the 100 point limit on Wargear taken by a model (so a master-crafted power weapon would count as 15 points towards the 100 point limit, not 30 points). Confusing, isn't it – but hey, nobody said playing this game was going to be simple!