A master-crafted weapon may re-roll one failed to-hit roll per turn. A master crafted ranged weapon may only re-roll hits for ranged attacks even if the weapon can also be used in close combat (such as pistols).
Power weapons ignore Armour Saves, and some may give a Strength bonus or wound on a fixed dice score regardless of the enemy's Toughness characteristic (as detailed in the appropriate army Codex).