3e SRD:Improved Critical
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Improved Critical [General]
Prerequisite: Proficient with weapon, base attack bonus +8 or higher.
Benefit: When using the weapon the character selected, the character's threat range is doubled.
Special: The character can gain this feat multiple times. The effects do not stack. Each time the character takes the feat, it applies to a new weapon. Note: "Keen" magic weapons also double their normal, nonmagical threat range. As with all doubled doublings, the result is triple.