From publisher blurb:
Mixed Magic and Firearms comes to Pathfinder with magic bolts and hot lead!
The sounds of battle ring different for those with arcane guns. The thunderous rhythm of gunpowder swelling in the ears and the crackle of spell energy causing arm hairs to stand on end is the heartbeat of the gun adept.
Carriers of magical blood that bonds with alchemical black powder, gun adepts are a rare breed of individuals. With the advance of technology to provide simple, often faulty, firearms, gun adepts are akin to sorcerers with innate magic that can only manifest their powers when holding a bonded fire iron. With this bond they are able to inscribe standard firearm ammunition with runes allowing it to be charged with arcane energy before pulling the trigger, resulting in a multitude of effects. Timing is everything to a gun adept and the pace of their casting ability is key. However if they were to burn out, they still hold in their hand a trusty sidearm that can get them till sunrise.
The gun adept material originally appears in Liber Xpansion.