Based on https://overframe.gg/build/76939/, with some tweaks for personal preference.
Change the elemental mods to adjust damage types for specific mission types:
General use: Venomous Clip, Polar Magazine, and Combustion Rounds for viral and heat (plus innate blast)
Isolation vaults and Orphix missions: Venomous Clip, Electrified Barrel, and Combustion Rounds for corrosive and heat (plus innate blast)
Eidolon hunting: [Electrified Barrel], [Combustion Rounds], and [Polar Magazine], for radiation and cold (plus innate blast)