When it comes to the mod loadout it is important to remember that a weapon should suit your playstyle.

I chose toxic as my progenitor element just because it's easier to make viral or corrosive. Honestly, any element works, but I will suggest to keep [Primed Charged Shell], [Primed Chilling Grasp], [Chilling Grasp] and [Blaze] in mind.

This weapon has an alt fire that uses 4 ammo.

When it comes to usability [Chilling Reload] reduces the reload time from 1.9s to 1.4s cutting 0.5s off your time. This may not seem like much, but with a magazine size of 4 you'll be reloading more often than you think. Assuming max fire rate button mashing you'll fire 4 rounds in 2.9s + 1.9s base reload totaling 4.8s of sustained DPS. 2.9s +1.4s [Chilling Reload] makes that 4.3s. On a more realistic playstyle, you'll be spending some time to aim as the weapon has one heck of a kick. About 1s per round is much more realistic and even then you might feel like that reload speed is slugish.

There are a number of builds that use [Primed Chilling Grasp] in place of [Chilling Reload] as they seem fine with the 1.9s reload time. If you're okay with that, feel free to replace.

The riven can be replaced with [Contagious Spread] to maintain the Viral element. If you choose corrosive you will end up with blast with [Blaze] + [Chilling Reload]. If you prefer to use [Primed Chilling Grasp], [Blaze] can be replaced with a dual status mod or [Vicious Spread]. [Hunter Munitions] is also a comfy mod.

Config A for a Viral + Radiation build.
Config B uses [Chilling Reload] , [Primed Tactical Pump] to maximize reload speed.

Happy Hunting.