General | Combat Stamina
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Crossbow Mastery (Combat)

Source Advanced Player's Guide pg. 157, Curse of the Crimson Throne Player's Guide pg. 10, Pathfinder Campaign Setting pg. 218
You can load crossbows with blinding speed and even fire them in melee with little fear of reprisal.

Prerequisites: Dex 15, Point-Blank Shot, Rapid Reload, Rapid Shot.

Benefit: The time required for you to reload any type of crossbow is reduced to a free action, regardless of the type of crossbow used. You can fire a crossbow as many times in a full attack action as you could attack if you were using a bow. Reloading a crossbow for the type of crossbow you chose when you took Rapid Reload no longer provokes attacks of opportunity.

Special: Starting at 6th level, a ranger with the archery combat style may select Crossbow Mastery as a combat style feat.

Combat Trick (from the Combat Stamina feat)

Source Pathfinder Unchained pg. 116
As long as you have at least 1 stamina point in your stamina pool, reloading any kind of crossbow no longer provokes attacks of opportunity.