Sniper's Creedo (3.5e Feat)
From D&D Wiki
Sniper's Creedo [General, Fighter]
Trained as a sniper there is one thing you value above all. Stealth.
Prerequisite: Must be taken at character creation. Must be proficient in a weapon with a range above 150 feet.
Benefit: When making a ranged attack from at least 150ft away without being noticed, gain bonus Sneak Attack damage as follows:
1d6 at level 1. 2d6 at level 4. 3d6 at level 8. 4d6 at level 12. 5d6 at level 16.6d6 at level 20.