SPAS-12 is a SG. SGs have one purpose: Armor tanking. Armor combined with Flash Ammo (which reduces damage taken by a flat -3) makes all other stats (including Health) largely irrelevant for SGs. Combining this with the fact that most SG Skills are too bad to acknowledge (as they are either DPS-based or have extremely low uptime) means that the Armor stat is the only thing that matters for most SGs.

SPAS-12 is given for free pre-raised at level 65 from Career Quests. While SPAS-12’s Skill doesn’t have any significant impact on combat, she still performs her role as a SG perfectly well, having 21 Armor. SPAS-12 also has a Mod that allows her to have some relevance later as well. As such, SPAS-12 is recommended as a first SG to raise.

When the wind is slow and the fire’s hot, SPAS-12 waits to see what rots. Oh how pretty, all the upgrades, this is G&K’s sacrifice.
Everyone’s favourite glutton, SPAS-12, gets herself an upgrade inspired by her favourite video game villain. Though, I guess she didn’t heed his desire to become f-in invincible, because her upgrade focuses entirely on enhancing her damage output.

Her new Skill 2 is Thorn Shield. This skill increases SPAS-12’s damage by the value of her current shield. Her Skill 1 also got a substantial upgrade, now with a 200% Damage increase, reduced Cooldown and Initial Cooldown, as well as another 1% increase to final damage for each point of Armor. On top of this, she receives the usual suite of stat upgrades and improved tiles.

It’s pretty clear that SPAS-12’s upgrade was designed with another particular doll in mind: SPAS-15. To briefly recap on SPAS-15, she gains a number of notes that can activate 1 of 3 effects based on her current team composition. SPAS-12 and SPAS-15 together automatically gives her 7 notes, so as long as you throw in at least one MG, you reach the 8 notes you would want to bolster SPAS-12.
At 8 notes or above, SPAS-15 will grant SPAS-12 with +100 Armor and a 200 HP Shield for 40 seconds. This translates into a flat 200 Damage buff to SPAS-12 as long as the shield is undamaged, on top of the 100 Armor further boosting the effects of SPAS-12’s upgraded Skill 1 and SPAS-15 reducing the cooldown on SPAS-12’s Skill 1 from her tiles. In the end, SPAS-12 can deal a surprisingly high amount of damage thanks to all these buffs.

It’s not all sunshine and burgers for SPAS-12 though: her damage output is reliant on her shield remaining undamaged, so you need to manage her position accordingly. There’s also still the simple fact that she’s a SG, which have naturally low Accuracy and Rate of Fire. She can be less useful against high Evasion or Armor targets and is very reliant on being able to hit multiple targets to keep up with DPS.

All in all, SPAS-12 MOD’s primary purpose is simple: no longer being a detriment to field with SPAS-15. This is particularly useful in rankings, where she can allow SPAS-15 to reach 8+ notes in situations where she couldn’t otherwise (such as Night maps that require a HG for vision), but is not very important for general content. Besides that, she’s not much of a tank but can put out some effective firepower in the right circumstances. MOD2 is recommended for her overall. Her Special Equipment is nice, but not imperative for her role.

SPAS-12 utilizes multiple types of damage increases, each with a different spot in damage calculation:

As a simplified example (ignoring AP and Crits): 

SPAS-12’s Skill sets her targets hit to 5. As stated in the Skill description, her Skill1 does not work with Birdshot nor Slug Ammo. As such, SPAS-12 should be using Buckshot Ammo.