You have the following options available to you, with this combination:
You may fire your gun, as a normal attack or full attack. This consumes ammo as normal, and deals damage as standard for the firearm in question if it hits.
You may cast a spell. While doing so, if the spell is a ranged touch, cone, line, or ray spell, you may choose to fire it through the firearm. Doing so applies the gun's enhancement bonus to the spell's attack roll or to its DC. This carries with it the cost of being broken, or exploding on you.
Additionally, if the spell is a ranged touch spell, you may choose to fire it through spellstrike. Doing so allows you to make a normal attack with your firearm, consuming ammo and applying firearm damage, but also applying the effects of the spell if it hits. It is probably possible to combine these two methods, with the heightened risk on if you roll a natural 1.
And finally, you may choose to use spell combat. Doing so is a full-round action, and allows you to cast a spell (with all considerations whether you are firing it through your gun) as well as make a normal attack with your gun. Note that if you are choosing to use spellstrike with your gun from the spell half of this, you must have some way to reload it in order to use it with the attack half.
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