Benefit: Make one attack roll. If the attack hits, the target takes damage from both bolts. Critical hits, sneak attack damage, and other precision-based damage only apply to the first bolt.
Drawback: Due to its size and weight, you take a –4 penalty on your attack roll if you’re proficient with it, or –8 if you’re not.
Load: Loading one bolt is a standard action; the Rapid Reload feat reduces this to a move action. Crossbow Mastery allows you to reload both bolts as a single move action.
Advanced Player’s Guide. Copyright 2010, Paizo Publishing, LLC; Author: Jason Bulmahn.