Main fire is a fully automatic rifle and alt-fire is a shotgun that consumes 15 ammo (normally 4 shots per magazine). Wildfire makes magazine capacity 78 and Primary Crux makes alt-fire consume 6 ammo instead, meaning you can shoot 13 alt-fire shots before reloading.
Galvanized Aptitude: mandatory base dmg +80% dmg per status. With 5 statuses we get +400% dmg potentially. Also adds bonus status chance
Galvanized Chamber : mandatory multishot up to +230%
Crit Delay + Vital Sense : mandatory crits
Primed Cryo Round + Malignant Force: makes viral and contributes to raw damage. A single viral proc multiplies damage by 2x, up to 4.25x at 10
Magnetic Capacity OR Wildfire: Magnetic Capacity gives magnetic status which is very strong against Thrax/Eximus as well as boosting magazine capacity. Wildfire gives heat dmg and status and boosts magazine capacity a bit.
Primed Shred: fire rate and punchthrough allows you to hit more enemies per bullet for clearing crowds
Vigilante Supplies: gives more ammo pickup and minor chance to boost crit tier. Can be rank 0
Primary Crux: gives ammo efficiency so the clip lasts longer if you spam and also allows you fire more alt-fire buckshots off, also gives status chance