One shots pretty much anything on the Steelpath with some Hunter Munitions time.

You can replace Heavy Caliber with Serration if you want it's just for bigger AoE. However for better damage replace with Bane mods. You can also replace Vile Acceleration with these options too if you're running any kind of firerate boosting such as Reinforced Bond or playing frames like Gauss/[Wisp]

Ammo pickup is just 1, so any ammo mutation at all will always give 1 so there's no point using any maxed mod. The nature of the weapon is you'll run out of ammo sometimes without further support (Carrier/Dispensary/etc.)

Toxin Progenitor to get Viral with just Primed Cryo Rounds, replace Vile Acceleration, Heavy Caliber or Primed Firestorm with Infected Clip if you have a different element.
If you aren't worried about Viral for your build then Cold Progenitor is best.

Also you obviously don't need 5 forma but you should do it for the Mastery anyway.