Radiation + Cold for alloy armor, with cold's extra damage to shields making up for radiation's weakness against it.

Swap [Cryo Rounds] out for [Hunter Munitions] (after adding a 5th forma) if you want to bleed high level ferrite armored targets like level 140 heavy gunners, it's better than pure damage against them but I prefer to just use another weapon instead of relying on slash procs from [Hunter Munitions] to take them out. I want most of my targets to die in a single shot, not just have a ~30% chance (taking crit chance and multishot into account) to get a slash proc.

[Cautious Shot] is too good for this weapon to give it up imo, although do note that it has a 90% chance to only reduce the stagger, not entirely eliminate it.

For this build you can use: radiation, fire, or electric for the natural bonus. More emphasis on electric since it allows you to make an alternate build of corrosive.