Please note: this is heavily based off of Maxximilius' archetype of the same name found in his Archetypes document. This is just my take on it.
Adepts of both black powder and explosive magic, some magi aren't content with the patience and risks involved in becoming spellslingers. Magi who favor versatility with guns and martial prowesses over arcane knowledge are called gunmages, pistol wielders blending the marksmanship of gunslingers with the magus's mastery of magic.
Weapon and Armor Proficiency: Gunmages are proficient with all simple weapons and one-handed firearms. He can cast magus spells while wearing light armor without incurring the normal arcane spell failure chance. Like any other arcane spellcaster, a magus wearing medium armor, heavy armor, or a shield incurs a chance of arcane spell failure if the spell in question has a somatic component. A multiclass magus still incurs the normal arcane spell failure chance for arcane spells received from other classes.
Gunsmith: This ability is identical to the gunslinger ability of the same name except the gunmage must take a pistol when he chooses a battered firearm at 1st level. This ability replaces cantrips, but the gunmage gains the detect magic and read magic cantrips and places them in his spellbook. He can cast either of these as 1st-level spells.
Gun Magician: At 1st level, a gunmage adds the following list of spells to the magus spell list. He must still select or discover them as normal before adding them to his spellbook:
1st: abundant ammunition, air bubble, peacebond, weaken powder
2nd: bullet shield, ricochet shot, destabilize powder, recoil fire, stabilize powder, thunder fire
4th: hostile juxtaposition, named bullet
6th: greater hostile juxtaposition, greater named bullet
This ability replaces greater spell access.
Pistolero Spell Combat (Ex): At 1st level, a gunmage can cast spells and wield his firearm at the same time. This functions much like two-weapon fighting, but the light off-hand weapon is a spell that is being cast. To use this ability, the magus must have one hand free (even if the spell being cast does not have somatic components), while wielding a one-handed firearm in the other hand. As a full-round action, he can make all of his attacks with his ranged weapon at a –2 penalty and can also cast any spell from the magus spell list with a casting time of 1 standard action (any attack roll made as part of this spell also takes this penalty). If he casts this spell defensively, he can decide to take an additional penalty on his attack rolls, up to his Intelligence bonus, and add the same amount as a circumstance bonus on his concentration check. If the check succeeds, the gunmage also does not provoke an attack of opportunity from making a ranged attack. If the check fails, the spell is wasted, but the attacks still take the penalty. A magus can choose to cast the spell first or make the weapon attacks first, but if he has more than one attack, he cannot cast the spell between weapon attacks. This ability replaces spell combat.
This ability is identical to the bladebound magus' black blade ability except the weapon is a one-handed firearm. This ability changes the Arcane Pool class feature and replaces the magus arcana gained at 3rd level.
Pistolero Spellstrike (Su): At 2nd level, whenever a gunmage casts a spell with a range of “touch” from the magus spell list, he can deliver the spell through any one-handed firearm he is wielding as part of a ranged attack as long as the target is within the first range increment of the weapon. Instead of the free melee touch attack normally allowed to deliver the spell, a magus can make one free ranged attack with his weapon (at his highest base attack bonus) as part of casting this spell. If successful, this ranged attack deals its normal damage as well as the effects of the spell. If the magus makes this attack in concert with spell combat, this melee attack takes all the penalties accrued by spell combat melee attacks. This attack uses the weapon’s critical range (20, 19–20, or 18–20 and modified by the keen weapon property or similar effects), but the spell effect only deals ×2 damage on a successful critical hit, while the weapon damage uses its own critical modifier. This ability replaces spellstrike.
Ebony Pistol: At 3rd level, a gunmage gains a powerful sentient weapon called an ebony pistol, which is always a one-handed firearm chosen by the gunmage. Despite the name, the weapon may be any one-handed firearm, and not just a pistol (subject to GM approval). A magus with this class feature cannot take the familiar magus arcana, and cannot have a familiar of any kind, even from another class. Instead of the normal arcane pool amount, the gunmage's arcane pool has a number of points equal to 1/3 his level (minimum 1) plus his Intelligence bonus.
Bonus Feat (Ex): At 5th level, and every six levels thereafter, a gunmage gains a bonus feat in addition to those gained from normal advancement. These bonus feats must be selected from those listed as combat, grit, or metamagic feats. He must meet the prerequisites for these feats as normal. This ability changes the magus' regular bonus feat class feature.
Pistol Training (Ex): At 7th level, the gunmage gains a bonus on one-handed firearms damage rolls equal to his Dexterity modifier, and when he misfires with a one-handed firearm, the misfire value increases by 2 instead of 4. At 13th level, a gunmage never misfires with a one-handed firearm. This ability replaces medium armor and heavy armor proficiency.
Gunmage's Grit (Ex): At 10th level, a gunmage gains a number of grit points equal to his Intelligence modifier (minimum 1) and gains the use of a single gunslinger deed. He can select any deed that a gunslinger of his magus level -3 could use. At 14th and 18th level, the gunmage gains another point of grit and another deed a gunslinger of his magus level -3 could use. If he already has levels in gunslinger, he gains a bonus to the maximum amount of grit he can have each day equal to his Intelligence modifier (if any) but gains no extra grit at the start of each day. This ability replaces fighter training.