Improved Critical

 
Prerequisite(s): Proficient with weapon, Base Attack Bonus 8+
 
Benefit: When using the weapon you selected, your threat range is doubled.
 
Special: 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.
 
Source: Players Handbook