D20:Improved Critical Feat

Choose one type of weapon.

Proficient with weapon, base attack bonus +8.

When using the weapon you selected, your threat range is doubled.

You can gain Improved Critical multiple times. The effects do not stack. Each time you take the feat, it applies to a new type of weapon.
This effect doesn?t stack with any other effect that expands the threat range of a weapon.
A fighter may select Improved Critical as one of his fighter bonus feats.

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System Reference Document -> Feats