Ninja (Fallout Supplement)
From D&D Wiki
When you attack the critters of the waste you have a better chance of really hurting them.
Prerequisite: Proficient with weapon, BAB +6
Benefit: When attacking with a weapon with which you are proficient the critical range for that weapon is doubled! You also get a +1 to your damage when you do crit (after damage adjustment).
Special: You may take Ninja twice, the second time it increases the critical range by 1 and the bonus damage by 2. I.E. A sword with a crit range of 19 - 20 would after taking Ninja once have a crit range of 17 - 20 with a plus 1 damage. The second time it would have a crit range of 16 - 20 with a plus 3 damage.