hiya. well honestly just pick whatever you want to use. if you want your character to be dual pistols and fired based, then firestrike as your main ability is the best choice, max everything in it as much as you can.
your whole build will revolve around getting the most out of firestrike, so you want to pick up some attack speed (with vindictive flame for example) and any form of resistance reduction you can get: blackwater cocktail for reduced res., flash bang for reducing DA and thus allowing for more crits; thermite mine for % fire res. debuff.
skills like stun jack or cannister bomb with its transmuter can give you some more CC if you feel the need for them. grenado can give you some really nice burst damage and mortar trap is nice if you want to have a lot of explosions!
if you want to choose the Inquisitor as your second mastery maybe take a look at the skills that are already knows (such as the DW skill) to maybe plan what you want from this mastery.
for devotions you want fire stuff obviously, id aim for ulzuins torch and well anything that you want/need for your build (dont forget to build up defenses, blast shield works great tbh)