aerilon
2019-11-17, 12:36 AM
Hello, everyone. This is Version 2 of this Mini-Guide to building a Gunmage - a Magus using a firearm. The first version was not JUST long, but lacking in structure and formatting. I've tried to clean it up a bit here.
I got the idea for this build in a game where I was playing an Eldritch Archer, and also the only prepared arcane caster in our party. And, coincidentally, I had decent attribute rolls, but not so great I would have felt like a standard archer was a real, viable choice. Rather than completely abandon my role - since the rest of the party was built already - I hit the internet to see how I could make it work. I looked at Kurald Galain's Guide to the Magus - a great resource - and at Serafina's "How to be Harry Dresden" or "Staff and Gun" guides.
They offered a lot of insight. In particular, they put me onto the idea of an Eldritch Archer with a gun. But I didn't agree with Serafina's choice of dipping Spellslinger. I'm not saying it isn't viable, let me be clear. But, at least for the game I was in, I didn't think it represented a good fit. And if I'm being frank, I really dislike the Spellslinger archetype. So, I decided to make a straight Eldritch Archer work. I will discuss, at the end of this post, alternatives to going straight Eldritch Archer Magus.
Here are the results.
Race: If we're not taking a level in Spellslinger, there's really only two race choices that makes sense for this build. That's Half-Elf or Elf. You need Weapon Proficiency, and the Half-Elf can take Ancestral Arms for that AND get the Elf favored Class Bonus of 1/6 of a New Magus Arcana to get a total of 3 extra Arcana over the life of the character (if your game goes that high). The Elf, obviously, can take the same FCB - but doesn't get the option to get Exotic Weapon Proficiency from an alternative Racial Trait. But that's okay, because a Cracked Opalescent White Pyramid Ioun Stone is only 1500 GP, so you can probably get one by Level 3 or so.
Attributes: Priority goes Int>Dex>Con>Str>Wis>Cha
You don't need as much Dex as most archers. The highest dex requirement you'll have to deal with is 19 for Improved Precise Shot, which you can't get until Level 15 when you've probably got a +6 Belt. As long as you have at least 13 when you start, you could hypothetically never put another point into Dexterity. Since you're going to want to be in the first range increment of your weapon to target Touch AC, it's not even as important for your accuracy as it is for other ranged combatants. This frees you up to focus on Intelligence, and be a Caster-focused Magus while still being capable of dealing weapon damage. It's true you don't have attribute to damage unless you multi-class (more on that later) but as we go forward I'll explain why that won't be as big of a problem for you as it is for others. Con because Fortitude Saves and Hit Points are important, Strength because carrying capacity and smart enemies will try to get into melee, and Wisdom because you don't want to blow a will save. Charisma is your dump stat.
A 15 point buy might look like this.
Half-Elf
Str 12 Dex 14 Con 12 Int 18 Wis 10 Cha 7
Traits: Top contenders are
Two-World Magic (Mending) if you don't have anyone in your party with the Mending cantrip.
Magical Lineage
Wayang Spellhunter
Roving Range (if using Pistol)
Yes, I pointed out Mending. This is because you don't get Quick Clear OR Gunsmithing to clear the "Broken" condition. Mending will do it. You can't use it in combat, but that's why you have a spell called "Jury-Rig" that you should always have prepared. If for some reason neither you nor anyone in your party has access to the Mending cantrip, try to see if your GM will let you buy Scrolls of Mending. They should be 12.5 GP a pop, and SOMEONE should have points in Use Magic Device.
Your Weapon: Pistol or Musket?
In truth, it's up to you. Firearms generally come with some well-known drawbacks. Reload times mean getting full attacks require some investments. But we have a way around that, albeit one that won't come online until Level 3 at the earliest, potentially Level 5 depending on how we prioritize things. What am I talking about? A feat combo consisting of Arcane Strike and Spell Cartridges. This feat allows you to substitute Spell Cartridges for your weapon damage, but more importantly, according to comments about the author's intent it also allows you to bypass the need to reload your weapon. All for the cost of a Swift Action. Bypassing the need for Rapid Reload entirely. Also opening up the option for dual-wielding pistols if you want to go that path.
The thing about Spell Cartridges is that it's poorly written because it says it deals "1d4" "for every five caster levels you have." I was fortunate - the GM of my game chose what I think is the most favorable interpretation of it, that it scales as Arcane Strike - but others might not. Depending on your GM, they might say it has no damage dice before you reach Caster Level 5 and therefore only delivers 1 point of damage (or just doesn't work), or that it has the minimum 1 damage dice until you reach Caster Level 10. Check with your GM on their interpretation. Despite its flaws, it overtakes weapon damage for both pistols and muskets as it scales. A pistol does 4.5 damage per bullet on average, but 2d4 for a force bullet is 5 on average. A musket does 6.5, but 3d4 is 7.5 on average. And depending on your GM's ruling, it will eventually scale up to either 4d4 or 5d4. Combined with Arcane Strike, this can represent an average of 8.5 or even 11 extra damage per shot. That's a lot of damage, considering you'll be putting 5 or 6 bullets out every full attack in the endgame.
The two main considerations I suggest you take into account when choosing whether you want to deliver your Force Bullets from a Pistol or a Musket is what you picture your endgame weapon being, and how far away from your targets you want to be. The gold standard, imo, for guns is the Pistol of the Infinite Sky. Not to disparage other abilities, but it 1: never misfires and 2: creates its own ammo. The first is big, but you can get it by enhancing your bonded weapon in the right way, but the second is huge because it allows you to still full attack on a round when you need to use your Swift for something other than Arcane Strike & Spell Cartridges. Shadowshooting or a Shadowcrafted Weapon can get you that as well, but allow Will saves to reduce your weapon damage to minimum. Thus, while these could serve as a stand-in, enemy Will saves eventually outgrow them. If you're in a home game and your GM allows you to replicate the "Infinite Sky" capabilities in a Musket, I prefer a Musket. If you're in a game like PFS, where it's strictly what's in the books, I lean more toward the pistol.
I mentioned dual-wielding pistols. I'll address it briefly here before I move on. What do you get from dual-wielding pistols, for the feat investment? An extra 3 attacks per round, at the endgame. I'll be brief and blunt. If you're going straight magus, I don't think it's worth the feat investment unless you're playing in a game with the Feat Tax Fix. Even then, I strongly urge you to put the extra feats toward your spellcasting and focus on a single weapon. If you REALLY want to dual-wield pistols, refer to the segment at the end of my post where I discuss alternative build ideas.
Feats:
I like to break this up into "Needed", "Recommended", and "I like these." The picks will be outlined assuming a normal game, without the Feat Tax Fix.
Need: Point Blank Shot, Precise Shot, Weapon Proficiency, Arcane Strike, Spell Cartridges, Deadly Aim, Rapid Shot, Intensify Spell
Recommended: Improved Precise Shot, Quicken Spell, Empower Spell, Spell Perfection, Riving Strike, Clustered Shots
Liked: Aerial Roll, Improved Critical, Spell Focus (Evocation), Greater Spell Focus (Evocation), Magic Trick (Floating Disk or Fireball)
What might my feat choices, and the order I would take them, look like on our Hypothetical Half-Elf? Well, in a game where the GM had Spell Cartridges scaling the way mine did, it might look like this.
1: Weapon Proficiency, Point Blank Shot. 3: Precise Shot. 5: Arcane Strike, Spell Cartridges. 7: Intensify Spell. 9: Deadly Aim. 11: Rapid Shot, Riving Strike. 13: Empower Spell. 15: Clustered Shots. 17: Metamagic Feat, Spell Perfection. 19: Improved Critical.
Before Level 5, I'd be using something other than my gun, most likely. Because not only is it important to be able to shoot into melee without the -4 from firing into melee, but the Spell Cartridge damage is less than either the Pistol or the Musket until it reaches at least 2d4. Which is one of the reasons I said an Elf not starting with Exotic Weapon Proficiency wasn't that bad.
What about a game where Spell Cartridges was interpreted more strictly, and got its first d4 at 5? I might put it off until later and use a different weapon at first, making the Elf and Ioun Stone option even more attractive, and my feat choices might look like this.
1: Point Blank Shot. 3: Precise Shot. 5: Intensify Spell, Arcane Strike. 7: Deadly Aim. 9: Riving Strike. 11: Spell Cartridges, Rapid Shot. 13: Empower Spell. 15: Clustered Shots. 17: Metamagic Feat, Spell Perfection. 19: Improved Critical.
You could certainly drop Riving Strike, but since you'll be using Arcane Strike regularly anyway, it gives you a nice debuff that can soften your targets up for your spells. And Improved Precise Shot isn't as necessary for you as it is for other ranged attackers, so it could be swapped out for something else. I could actually see dropping Quicken Spell, believe it or not, as your Swift Action is worth as much as 50-60 average extra damage per round at those levels if used for Arcane Strike & Spell Cartridges. That's almost as much damage as a 20d6 spell dealing average damage if your target doesn't make their save, and is comparable to a 30d6 spell's average damage if the target did save. As for Improved Critical - it's only a 5% increase in your chances, but if your weapon crits, not only is it a x4 modifier, but your spells crit too.
Magus Arcana: This list will be chosen from Paizo content only, despite that there are some great 3pp options.
If you're an Elf or Half-Elf and you take the Favored Class Bonus, you'll end up with 9 Arcana. There are a few you should DEFINITELY take, past that it's sort of dealer's choice. First thing's first - skip all the melee focused ones, you're not a melee Magus.
Recommended Arcana
Level 3: Familiar - because I like familiars, they can serve as scouts, and potentially use wands.
Level 6: Free
Level 6 (Bonus): Free
Level 9: Reach Spellstrike - add more options for your ranged spellstrike.
Level 12: Free
Level 12 (Bonus): Free
Level 15: Bane Blade
Level 18: Quickened Magic
Level 18 (Bonus): Spell Blending
Why not Devoted Blade? Honestly, because by the time you can get it, you probably already have (or will at the next level) a +5 weapon either inherently, or by virtue of being able to use your Arcane Pool to make it one. And if it's not for bypassing the DR, I'd rather use those 2 points of enhancement bonus to add two different other types of energy damage dice. I like Spell Scars for a Level 6 slot, and think it pairs well with Scroll Mastery, but not everyone would agree with me. You could take Spell Blending again at 12. I like to wait until I've got access to multiple spell levels to pick two spells from instead of just one from the highest level before taking it, but there's no reason you couldn't take it twice if there are lower level spells you want. What about hasted assault? Because you learn Haste and you have Spell Recall.
So what do we end up with? A magus with a gun, ideally firing a weapon that never misfires - even on a 1 - and creates its own ammo. The ability to use Arcane Strike & Spell Cartridges to ramp up our damage per shot by 8-11 points, and our primary focus on our casting abilities. At level 16, you're looking at a (BAB 12, Dex 5 from base and a Belt +6, +5 from our weapon, +2 from Bane, likely 1 from Point Blank, a probable 4 from buffs if you've got at least Bless, Haste, and Heroism or Good Hope in the party, -8 for Spell Combat, Rapid Shot, and Deadly Aim) +21/+21/+21/+21/+16/+11 if you're using Spellstrike. VS Touch AC. And each round is doing, if you're using Arcane Strike - and let's assume the worst case scenario on the scaling of Spell Cartridges - (Spell Cartridges 7.5 + Arcane Strike 4 + Deadly Aim 8 + Weapon 5 + Bane Enhancement Bonus 2 + 2 Heroism or Good Hope + Point Blank 1, shock/flaming/frost from your arcane pool for 3d6 avg 10.5 extra and another 2d6 from Bane for an extra 7) 47 points of damage per shot. Not bad, not bad at all. And while it does get better at later levels, this build is still able to contribute at low levels.
Other considerations or build concepts.
So, I dislike the Spellslinger, and that was a big part of building this character and writing this overview of a way to bring the concept to life. But the Spellslinger does offer ONE thing that I acknowledge is actually of MORE benefit to this Magus than to a less caster-oriented Magus. The ability to add the weapon's enhancement bonus to the DC of your spells, thus making it harder to save against your spells. Enemy saving throws mean a lot of your damage will get lost as you rise in levels, and while this build can pump Intelligence more than other Magi can, we still don't have the same opportunities to ramp up our DCs the way a pure caster does. The Spellslinger, through the Arcane Gun, lets us close some of the gap. If we use Riving Strike, and if one of our friendly Power-Attacking Melee allies has Cornugon Smash (which should be a staple on Power Attack builds, in my opinion) that could be a huge help, even more if someone has a Cruel weapon. If I was going to dip Spellslinger, I would definitely pick the Elf over the Half-Elf, but it wouldn't significantly change the build much beyond that.
I mentioned I would come back to dual-wielding pistols. To be honest, I don't consider it worth the feat investment with a straight Magus. But with a multi-class, especially one that gets dexterity to damage, my opinion changes. I'd probably only do this in a home game with the feat tax fix, because it's STILL very feat-intensive, but 3 levels of Trench Fighter would get you 2 Bonus Feats to help with the feat cost, and Dexterity to damage. Adding Dexterity to damage when you're getting 6 shots a round? If your dex bonus is +8, that's up to 48 extra damage. If you're getting 9 shots, while dual-wielding two pistols? That's almost 80 extra points of damage per round. My personal opinion is it wouldn't be worth giving up 5 levels to do this, you'd be giving up too much magic. Specifically, you need at least 16 levels of Magus to even get access to 6th level spells, even if you're willing to have fewer spell slots.
The single highest damage build I made AT LEVEL 20, according to the spreadsheet I have to run DPR calculations, was actually a Spellslinger1/Trench Fighter3/Magus X. The thing is, the damage didn't start substantially outpacing a single weapon until late-game, we're talking level 15+. What made this build more damaging was threefold - the extra damage from dexterity coming from Trench Fighter, that extra damage being applied on top of the normal damage to 9 shots per round instead of 6, and the greater likelihood that targets of your spells would fail their saving throws. This build's damage per round potential came reasonably close to a pouncing, full-attacking bloodrager with 42 strength, raging brutality, polymorphed via monstrous physique into a form that allowed it to use manufactured weapons and still get 4 natural attacks. And easily put out enough to end a campaign boss in a single round by itself. In the end, that's one of the reasons I chose not to play it - a desire for the game to be fun for everyone. A downside was that it was very rigid. Here's how it would look, assuming Spell Cartridges scales 1d4 at Caster Level 5, up to 4d4 at Caster Level 20 (you would need several ways to raise your caster level, btw).
Spellslinger1>Magus2>12, Trench Fighter 13-15, Magus 16-20.
Elf (15 point buy)
Str 12, Dex 16, Con 10, Int 18, Wis 10, Cha 7
1: (Class) Weapon Proficiency, Point Blank Shot. 3: Precise Shot. 5: Intensify Spell. 6: Arcane Strike. 7: Deadly Aim. 9: Spell Cartridges. 11: Empower Spell. 12: Rapid Shot. 13: Metamagic Feat, Two Weapon Fighting. 14: Improved Two Weapon Fighting. 15: Greater Two Weapon Fighting. 17: Spell Perfection. 19: Clustered Shots.
Compare at Level 16: BAB 12, Dex 6, Weapon 5, Buffs 4, Point Blank 1, Rapid Shot, Spell Combat, Deadly Aim, and TWF -12 = +16/+16/+16/+16/+16/+11/+11/+6/+6
Damage (assuming you're at least only -1 caster level vs character level): Cartridges 7.5, Deadly Aim 8, Buff 2, Point Blank 1, Arcane Strike 4, Dexterity 6, Weapon 5, Shock/Flaming/Frost 10.5 = 44 average damage per hit on one gun, and 32.5 on the other, and you're putting out 9 shots vs touch AC. And this build doesn't have Bane Blade yet, as it hasn't reached Magus 15. If you're doing this, and your GM allows it, I highly recommend the 3pp Split Arcana. This build ends up outpacing the others in pure damage output from weapons, but it has sacrificed spellcasting (there's a reason I delayed the Trench Fighter levels until later, you don't want to put off access to your early magic) to do it, and it has very little wiggle room to work with. Even with the Feat Tax Fix, the only things you can really drop are Point Blank Shot and Greater Two Weapon Fighting - which I'd swap for Spell Focus & Greater Spell Focus. I'd still choose Arcane Strike and Spell Cartridges, as they represent a huge amount of damage that you can deliver consistently, without using up your fewer high level spell slots. I didn't specifically choose Quicken Spell because not only is Arcane Strike & Spell Cartridges usually going to be a better use of your Swift Action after the first round (when you apply your Arcane Pool bonuses) for consistent damage output, you also just don't have many Level 6 Spell Slots. You would need to hit 38 Intelligence to get a 4th spell slot (1 normally, the rest bonus), which is hard to hit. Even starting at 20 you would only likely get at most 5 from leveling, 5 from Inherent bonuses, and 6 from an Enhancement bonus.
I still prefer a single weapon focused build, and I really disliked the Spellslinger. The difference in average weapon-based damage per round between the above build and a single-weapon focused build is that you drop from being able to kill a campaign boss by yourself in a single round to being able to drop them to half or 1/3 health instead. Before you cast a spell. And with Spell Perfection (and especially if you retrain Magical Lineage or Wayang Spellhunter to cover it) Hellfire Ray can cover the difference.
What do your Spell DCs look like? Well, at Level 16, a 5th level spell would be 10 + 5 + Int Mod (18 + 4Lvl + 6Headband) 9 = 24 before you add any debuffs to the target - such as Riving Strike, or your hoped for Ally's Intimidate effect. Against an equal CR enemy - according to the stats I looked up on d20pfsrd the highest Reflex save (because most of your blast spells that can be used for Spellstrike seem to use Reflex) for that CR is +18, but most of them were closer to 12-14 - around a 50% chance to stick full damage is not too bad, even if it doesn't stack up to a full Wizard. And once they've been debuffed it looks a lot better, depending on how many debuff effects your party can stack your chance to land full damage could be closer to 80%. And, of course, your DCs will only get better once you get your Inherent bonus (I figure on Level 18 for that) from a Tome or from someone in your party getting Wish. You could swap Improved Critical for Spell Focus to get another +2 when you get to that point (assuming we're using it with our Spell Perfection choice). And that's without taking the level of Spellslinger. Since, even up to CR 22 (CR+2 for a level 20 character) the highest Reflex Save listed in the source I used is only 27 (and the averages are much lower, around 20 even at the levels where it is that high) and your DCs can hit 33 before debuffs without Arcane Gun, against most targets you'll probably have at least a 50/50 chance before debuffing, I prefer going straight Magus.
In truth, I think the absolutely optimized choice for maxing DCs with this build is an Elf, with the Proficiency coming from an Ioun Stone allowing you to treat your chosen firearm as a martial weapon. Swapping to the alternative racial trait giving you Spell Focus as a bonus feat, and taking Greater Spell Focus instead of Improved Critical.
Thoughts, comments, criticisms, and feedback welcome. Credit to Serafina and Kurald Galain as the inspiration for this build and this treatment.
I got the idea for this build in a game where I was playing an Eldritch Archer, and also the only prepared arcane caster in our party. And, coincidentally, I had decent attribute rolls, but not so great I would have felt like a standard archer was a real, viable choice. Rather than completely abandon my role - since the rest of the party was built already - I hit the internet to see how I could make it work. I looked at Kurald Galain's Guide to the Magus - a great resource - and at Serafina's "How to be Harry Dresden" or "Staff and Gun" guides.
They offered a lot of insight. In particular, they put me onto the idea of an Eldritch Archer with a gun. But I didn't agree with Serafina's choice of dipping Spellslinger. I'm not saying it isn't viable, let me be clear. But, at least for the game I was in, I didn't think it represented a good fit. And if I'm being frank, I really dislike the Spellslinger archetype. So, I decided to make a straight Eldritch Archer work. I will discuss, at the end of this post, alternatives to going straight Eldritch Archer Magus.
Here are the results.
Race: If we're not taking a level in Spellslinger, there's really only two race choices that makes sense for this build. That's Half-Elf or Elf. You need Weapon Proficiency, and the Half-Elf can take Ancestral Arms for that AND get the Elf favored Class Bonus of 1/6 of a New Magus Arcana to get a total of 3 extra Arcana over the life of the character (if your game goes that high). The Elf, obviously, can take the same FCB - but doesn't get the option to get Exotic Weapon Proficiency from an alternative Racial Trait. But that's okay, because a Cracked Opalescent White Pyramid Ioun Stone is only 1500 GP, so you can probably get one by Level 3 or so.
Attributes: Priority goes Int>Dex>Con>Str>Wis>Cha
You don't need as much Dex as most archers. The highest dex requirement you'll have to deal with is 19 for Improved Precise Shot, which you can't get until Level 15 when you've probably got a +6 Belt. As long as you have at least 13 when you start, you could hypothetically never put another point into Dexterity. Since you're going to want to be in the first range increment of your weapon to target Touch AC, it's not even as important for your accuracy as it is for other ranged combatants. This frees you up to focus on Intelligence, and be a Caster-focused Magus while still being capable of dealing weapon damage. It's true you don't have attribute to damage unless you multi-class (more on that later) but as we go forward I'll explain why that won't be as big of a problem for you as it is for others. Con because Fortitude Saves and Hit Points are important, Strength because carrying capacity and smart enemies will try to get into melee, and Wisdom because you don't want to blow a will save. Charisma is your dump stat.
A 15 point buy might look like this.
Half-Elf
Str 12 Dex 14 Con 12 Int 18 Wis 10 Cha 7
Traits: Top contenders are
Two-World Magic (Mending) if you don't have anyone in your party with the Mending cantrip.
Magical Lineage
Wayang Spellhunter
Roving Range (if using Pistol)
Yes, I pointed out Mending. This is because you don't get Quick Clear OR Gunsmithing to clear the "Broken" condition. Mending will do it. You can't use it in combat, but that's why you have a spell called "Jury-Rig" that you should always have prepared. If for some reason neither you nor anyone in your party has access to the Mending cantrip, try to see if your GM will let you buy Scrolls of Mending. They should be 12.5 GP a pop, and SOMEONE should have points in Use Magic Device.
Your Weapon: Pistol or Musket?
In truth, it's up to you. Firearms generally come with some well-known drawbacks. Reload times mean getting full attacks require some investments. But we have a way around that, albeit one that won't come online until Level 3 at the earliest, potentially Level 5 depending on how we prioritize things. What am I talking about? A feat combo consisting of Arcane Strike and Spell Cartridges. This feat allows you to substitute Spell Cartridges for your weapon damage, but more importantly, according to comments about the author's intent it also allows you to bypass the need to reload your weapon. All for the cost of a Swift Action. Bypassing the need for Rapid Reload entirely. Also opening up the option for dual-wielding pistols if you want to go that path.
The thing about Spell Cartridges is that it's poorly written because it says it deals "1d4" "for every five caster levels you have." I was fortunate - the GM of my game chose what I think is the most favorable interpretation of it, that it scales as Arcane Strike - but others might not. Depending on your GM, they might say it has no damage dice before you reach Caster Level 5 and therefore only delivers 1 point of damage (or just doesn't work), or that it has the minimum 1 damage dice until you reach Caster Level 10. Check with your GM on their interpretation. Despite its flaws, it overtakes weapon damage for both pistols and muskets as it scales. A pistol does 4.5 damage per bullet on average, but 2d4 for a force bullet is 5 on average. A musket does 6.5, but 3d4 is 7.5 on average. And depending on your GM's ruling, it will eventually scale up to either 4d4 or 5d4. Combined with Arcane Strike, this can represent an average of 8.5 or even 11 extra damage per shot. That's a lot of damage, considering you'll be putting 5 or 6 bullets out every full attack in the endgame.
The two main considerations I suggest you take into account when choosing whether you want to deliver your Force Bullets from a Pistol or a Musket is what you picture your endgame weapon being, and how far away from your targets you want to be. The gold standard, imo, for guns is the Pistol of the Infinite Sky. Not to disparage other abilities, but it 1: never misfires and 2: creates its own ammo. The first is big, but you can get it by enhancing your bonded weapon in the right way, but the second is huge because it allows you to still full attack on a round when you need to use your Swift for something other than Arcane Strike & Spell Cartridges. Shadowshooting or a Shadowcrafted Weapon can get you that as well, but allow Will saves to reduce your weapon damage to minimum. Thus, while these could serve as a stand-in, enemy Will saves eventually outgrow them. If you're in a home game and your GM allows you to replicate the "Infinite Sky" capabilities in a Musket, I prefer a Musket. If you're in a game like PFS, where it's strictly what's in the books, I lean more toward the pistol.
I mentioned dual-wielding pistols. I'll address it briefly here before I move on. What do you get from dual-wielding pistols, for the feat investment? An extra 3 attacks per round, at the endgame. I'll be brief and blunt. If you're going straight magus, I don't think it's worth the feat investment unless you're playing in a game with the Feat Tax Fix. Even then, I strongly urge you to put the extra feats toward your spellcasting and focus on a single weapon. If you REALLY want to dual-wield pistols, refer to the segment at the end of my post where I discuss alternative build ideas.
Feats:
I like to break this up into "Needed", "Recommended", and "I like these." The picks will be outlined assuming a normal game, without the Feat Tax Fix.
Need: Point Blank Shot, Precise Shot, Weapon Proficiency, Arcane Strike, Spell Cartridges, Deadly Aim, Rapid Shot, Intensify Spell
Recommended: Improved Precise Shot, Quicken Spell, Empower Spell, Spell Perfection, Riving Strike, Clustered Shots
Liked: Aerial Roll, Improved Critical, Spell Focus (Evocation), Greater Spell Focus (Evocation), Magic Trick (Floating Disk or Fireball)
What might my feat choices, and the order I would take them, look like on our Hypothetical Half-Elf? Well, in a game where the GM had Spell Cartridges scaling the way mine did, it might look like this.
1: Weapon Proficiency, Point Blank Shot. 3: Precise Shot. 5: Arcane Strike, Spell Cartridges. 7: Intensify Spell. 9: Deadly Aim. 11: Rapid Shot, Riving Strike. 13: Empower Spell. 15: Clustered Shots. 17: Metamagic Feat, Spell Perfection. 19: Improved Critical.
Before Level 5, I'd be using something other than my gun, most likely. Because not only is it important to be able to shoot into melee without the -4 from firing into melee, but the Spell Cartridge damage is less than either the Pistol or the Musket until it reaches at least 2d4. Which is one of the reasons I said an Elf not starting with Exotic Weapon Proficiency wasn't that bad.
What about a game where Spell Cartridges was interpreted more strictly, and got its first d4 at 5? I might put it off until later and use a different weapon at first, making the Elf and Ioun Stone option even more attractive, and my feat choices might look like this.
1: Point Blank Shot. 3: Precise Shot. 5: Intensify Spell, Arcane Strike. 7: Deadly Aim. 9: Riving Strike. 11: Spell Cartridges, Rapid Shot. 13: Empower Spell. 15: Clustered Shots. 17: Metamagic Feat, Spell Perfection. 19: Improved Critical.
You could certainly drop Riving Strike, but since you'll be using Arcane Strike regularly anyway, it gives you a nice debuff that can soften your targets up for your spells. And Improved Precise Shot isn't as necessary for you as it is for other ranged attackers, so it could be swapped out for something else. I could actually see dropping Quicken Spell, believe it or not, as your Swift Action is worth as much as 50-60 average extra damage per round at those levels if used for Arcane Strike & Spell Cartridges. That's almost as much damage as a 20d6 spell dealing average damage if your target doesn't make their save, and is comparable to a 30d6 spell's average damage if the target did save. As for Improved Critical - it's only a 5% increase in your chances, but if your weapon crits, not only is it a x4 modifier, but your spells crit too.
Magus Arcana: This list will be chosen from Paizo content only, despite that there are some great 3pp options.
If you're an Elf or Half-Elf and you take the Favored Class Bonus, you'll end up with 9 Arcana. There are a few you should DEFINITELY take, past that it's sort of dealer's choice. First thing's first - skip all the melee focused ones, you're not a melee Magus.
Recommended Arcana
Level 3: Familiar - because I like familiars, they can serve as scouts, and potentially use wands.
Level 6: Free
Level 6 (Bonus): Free
Level 9: Reach Spellstrike - add more options for your ranged spellstrike.
Level 12: Free
Level 12 (Bonus): Free
Level 15: Bane Blade
Level 18: Quickened Magic
Level 18 (Bonus): Spell Blending
Why not Devoted Blade? Honestly, because by the time you can get it, you probably already have (or will at the next level) a +5 weapon either inherently, or by virtue of being able to use your Arcane Pool to make it one. And if it's not for bypassing the DR, I'd rather use those 2 points of enhancement bonus to add two different other types of energy damage dice. I like Spell Scars for a Level 6 slot, and think it pairs well with Scroll Mastery, but not everyone would agree with me. You could take Spell Blending again at 12. I like to wait until I've got access to multiple spell levels to pick two spells from instead of just one from the highest level before taking it, but there's no reason you couldn't take it twice if there are lower level spells you want. What about hasted assault? Because you learn Haste and you have Spell Recall.
So what do we end up with? A magus with a gun, ideally firing a weapon that never misfires - even on a 1 - and creates its own ammo. The ability to use Arcane Strike & Spell Cartridges to ramp up our damage per shot by 8-11 points, and our primary focus on our casting abilities. At level 16, you're looking at a (BAB 12, Dex 5 from base and a Belt +6, +5 from our weapon, +2 from Bane, likely 1 from Point Blank, a probable 4 from buffs if you've got at least Bless, Haste, and Heroism or Good Hope in the party, -8 for Spell Combat, Rapid Shot, and Deadly Aim) +21/+21/+21/+21/+16/+11 if you're using Spellstrike. VS Touch AC. And each round is doing, if you're using Arcane Strike - and let's assume the worst case scenario on the scaling of Spell Cartridges - (Spell Cartridges 7.5 + Arcane Strike 4 + Deadly Aim 8 + Weapon 5 + Bane Enhancement Bonus 2 + 2 Heroism or Good Hope + Point Blank 1, shock/flaming/frost from your arcane pool for 3d6 avg 10.5 extra and another 2d6 from Bane for an extra 7) 47 points of damage per shot. Not bad, not bad at all. And while it does get better at later levels, this build is still able to contribute at low levels.
Other considerations or build concepts.
So, I dislike the Spellslinger, and that was a big part of building this character and writing this overview of a way to bring the concept to life. But the Spellslinger does offer ONE thing that I acknowledge is actually of MORE benefit to this Magus than to a less caster-oriented Magus. The ability to add the weapon's enhancement bonus to the DC of your spells, thus making it harder to save against your spells. Enemy saving throws mean a lot of your damage will get lost as you rise in levels, and while this build can pump Intelligence more than other Magi can, we still don't have the same opportunities to ramp up our DCs the way a pure caster does. The Spellslinger, through the Arcane Gun, lets us close some of the gap. If we use Riving Strike, and if one of our friendly Power-Attacking Melee allies has Cornugon Smash (which should be a staple on Power Attack builds, in my opinion) that could be a huge help, even more if someone has a Cruel weapon. If I was going to dip Spellslinger, I would definitely pick the Elf over the Half-Elf, but it wouldn't significantly change the build much beyond that.
I mentioned I would come back to dual-wielding pistols. To be honest, I don't consider it worth the feat investment with a straight Magus. But with a multi-class, especially one that gets dexterity to damage, my opinion changes. I'd probably only do this in a home game with the feat tax fix, because it's STILL very feat-intensive, but 3 levels of Trench Fighter would get you 2 Bonus Feats to help with the feat cost, and Dexterity to damage. Adding Dexterity to damage when you're getting 6 shots a round? If your dex bonus is +8, that's up to 48 extra damage. If you're getting 9 shots, while dual-wielding two pistols? That's almost 80 extra points of damage per round. My personal opinion is it wouldn't be worth giving up 5 levels to do this, you'd be giving up too much magic. Specifically, you need at least 16 levels of Magus to even get access to 6th level spells, even if you're willing to have fewer spell slots.
The single highest damage build I made AT LEVEL 20, according to the spreadsheet I have to run DPR calculations, was actually a Spellslinger1/Trench Fighter3/Magus X. The thing is, the damage didn't start substantially outpacing a single weapon until late-game, we're talking level 15+. What made this build more damaging was threefold - the extra damage from dexterity coming from Trench Fighter, that extra damage being applied on top of the normal damage to 9 shots per round instead of 6, and the greater likelihood that targets of your spells would fail their saving throws. This build's damage per round potential came reasonably close to a pouncing, full-attacking bloodrager with 42 strength, raging brutality, polymorphed via monstrous physique into a form that allowed it to use manufactured weapons and still get 4 natural attacks. And easily put out enough to end a campaign boss in a single round by itself. In the end, that's one of the reasons I chose not to play it - a desire for the game to be fun for everyone. A downside was that it was very rigid. Here's how it would look, assuming Spell Cartridges scales 1d4 at Caster Level 5, up to 4d4 at Caster Level 20 (you would need several ways to raise your caster level, btw).
Spellslinger1>Magus2>12, Trench Fighter 13-15, Magus 16-20.
Elf (15 point buy)
Str 12, Dex 16, Con 10, Int 18, Wis 10, Cha 7
1: (Class) Weapon Proficiency, Point Blank Shot. 3: Precise Shot. 5: Intensify Spell. 6: Arcane Strike. 7: Deadly Aim. 9: Spell Cartridges. 11: Empower Spell. 12: Rapid Shot. 13: Metamagic Feat, Two Weapon Fighting. 14: Improved Two Weapon Fighting. 15: Greater Two Weapon Fighting. 17: Spell Perfection. 19: Clustered Shots.
Compare at Level 16: BAB 12, Dex 6, Weapon 5, Buffs 4, Point Blank 1, Rapid Shot, Spell Combat, Deadly Aim, and TWF -12 = +16/+16/+16/+16/+16/+11/+11/+6/+6
Damage (assuming you're at least only -1 caster level vs character level): Cartridges 7.5, Deadly Aim 8, Buff 2, Point Blank 1, Arcane Strike 4, Dexterity 6, Weapon 5, Shock/Flaming/Frost 10.5 = 44 average damage per hit on one gun, and 32.5 on the other, and you're putting out 9 shots vs touch AC. And this build doesn't have Bane Blade yet, as it hasn't reached Magus 15. If you're doing this, and your GM allows it, I highly recommend the 3pp Split Arcana. This build ends up outpacing the others in pure damage output from weapons, but it has sacrificed spellcasting (there's a reason I delayed the Trench Fighter levels until later, you don't want to put off access to your early magic) to do it, and it has very little wiggle room to work with. Even with the Feat Tax Fix, the only things you can really drop are Point Blank Shot and Greater Two Weapon Fighting - which I'd swap for Spell Focus & Greater Spell Focus. I'd still choose Arcane Strike and Spell Cartridges, as they represent a huge amount of damage that you can deliver consistently, without using up your fewer high level spell slots. I didn't specifically choose Quicken Spell because not only is Arcane Strike & Spell Cartridges usually going to be a better use of your Swift Action after the first round (when you apply your Arcane Pool bonuses) for consistent damage output, you also just don't have many Level 6 Spell Slots. You would need to hit 38 Intelligence to get a 4th spell slot (1 normally, the rest bonus), which is hard to hit. Even starting at 20 you would only likely get at most 5 from leveling, 5 from Inherent bonuses, and 6 from an Enhancement bonus.
I still prefer a single weapon focused build, and I really disliked the Spellslinger. The difference in average weapon-based damage per round between the above build and a single-weapon focused build is that you drop from being able to kill a campaign boss by yourself in a single round to being able to drop them to half or 1/3 health instead. Before you cast a spell. And with Spell Perfection (and especially if you retrain Magical Lineage or Wayang Spellhunter to cover it) Hellfire Ray can cover the difference.
What do your Spell DCs look like? Well, at Level 16, a 5th level spell would be 10 + 5 + Int Mod (18 + 4Lvl + 6Headband) 9 = 24 before you add any debuffs to the target - such as Riving Strike, or your hoped for Ally's Intimidate effect. Against an equal CR enemy - according to the stats I looked up on d20pfsrd the highest Reflex save (because most of your blast spells that can be used for Spellstrike seem to use Reflex) for that CR is +18, but most of them were closer to 12-14 - around a 50% chance to stick full damage is not too bad, even if it doesn't stack up to a full Wizard. And once they've been debuffed it looks a lot better, depending on how many debuff effects your party can stack your chance to land full damage could be closer to 80%. And, of course, your DCs will only get better once you get your Inherent bonus (I figure on Level 18 for that) from a Tome or from someone in your party getting Wish. You could swap Improved Critical for Spell Focus to get another +2 when you get to that point (assuming we're using it with our Spell Perfection choice). And that's without taking the level of Spellslinger. Since, even up to CR 22 (CR+2 for a level 20 character) the highest Reflex Save listed in the source I used is only 27 (and the averages are much lower, around 20 even at the levels where it is that high) and your DCs can hit 33 before debuffs without Arcane Gun, against most targets you'll probably have at least a 50/50 chance before debuffing, I prefer going straight Magus.
In truth, I think the absolutely optimized choice for maxing DCs with this build is an Elf, with the Proficiency coming from an Ioun Stone allowing you to treat your chosen firearm as a martial weapon. Swapping to the alternative racial trait giving you Spell Focus as a bonus feat, and taking Greater Spell Focus instead of Improved Critical.
Thoughts, comments, criticisms, and feedback welcome. Credit to Serafina and Kurald Galain as the inspiration for this build and this treatment.