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Source: Player's Handbook

Description: You deal extra damage when using a weapon you are specialized with

Prerequisites: Weapon Focus, Weapon Specialization, Base Attack Bonus +12, Weapon Proficiency, Greater Weapon Focus

You gain a +2 bonus on all damage rolls you make using the selected weapon. This bonus stacks with other bonuses on damage rolls, including the one from Weapon Specialization.

Special

You can gain Greater Weapon Specialization multiple times. Its effects do not stack. Each time you take the feat, it applies to a new type of weapon. A fighter may select Greater Weapon Specialization as one of his fighter bonus feats.