The combination of Viral and Slash is ridiculously effective.

The difference for me on Shotgun builds is whether they need added fire rate or not. This gun really benefits from extra fire-rate, so since I only have one open elemental damage slot, I went with Toxin for the progenitor element. That way I can use Primed Chilling Grasp to make Viral, or replace it with Primed Charged Shell to make Corrosive, or Contagious Spread for a pure Toxin Anti-Corpus build.