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Casterbane Shot (Combat, Grit)

Source Inner Sea Combat pg. 17
Life in the Mana Wastes, where magic has unpredictable and often fatal effects, has taught you to disable spellcasters with your firearms.

Prerequisites: Amateur GunslingerUC or the grit class feature, Point-Blank Shot, Precise Shot, base attack bonus +4.

Benefit: If you ready a ranged attack with a firearm to attack a spellcaster when he begins casting a spell and successfully hit your opponent, you can expend 1 grit point to increase the target’s concentration check DC for being injured while casting by 5.

Combat Trick (from the Combat Stamina feat)

Source Weapon Master's Handbook pg. 24
When using Casterbane Shot, you can spend up to 5 stamina points to increase the DC of the concentration check imposed by the feat by an amount equal to the number of stamina points spent.