Better Criticals (Fallout Supplement)
From D&D Wiki
BETTER CRITICALS [Combat]
You've performed enough brutal killings to know how to cause the most damage.
Prerequisite: BAB +8, Proficient with weapon.
Benefit: The critical threat range for a weapon you are proficient with is double.
Special: This feat may be taken a second time for the same weapon once your BAB is +10 and provides a +1 critical damage multiplier, so a weapon with a x2 crit multiplier would become x3. Or you can take it for another type of weapon.