Does the attack bonus from a Masterwork weapon stack with the attack bonus from Masterwork ammunition?
For example, does the +1 attack bonus of a Masterwork Arrow stack with the +1 attack bonus of Masterwork Short or Long Bow?
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As per Pathfinder's masterwork weapon rules:
A masterwork weapon is a finely crafted version of a normal weapon. Wielding it provides a +1 enhancement bonus on attack rolls. [...] The enhancement bonus of masterwork ammunition does not stack with any enhancement bonus of the projectile weapon firing it.
Which is in keeping with the general rules for magical weapons and ammunition:
Ranged Weapons and Ammunition: The enhancement bonus from a ranged weapon does not stack with the enhancement bonus from ammunition. Only the higher of the two enhancement bonuses applies.
The bonus to attack rolls granted by masterwork equipment is an enhancement bonus, and the enhancement bonus of masterwork ammunition explicitly does not stack with the launcher's enhancement bonus, so unfortunately you get no benefit by using masterwork ammunition if your launcher is already masterwork or better.
From the description of masterwork weapons:
Wielding it provides a +1 enhancement bonus on attack rolls. [...]
Masterwork ammunition is damaged (effectively destroyed) when used. The enhancement bonus of masterwork ammunition does not stack with any enhancement bonus of the projectile weapon firing it.