This is a gun I've taken to experimenting to after a while. I've always liked the gun and its functionalities. An automatic machine gun with an [Arca Plasmor] like alternate fire. A while back I tried something and..... oh dear.

Remove the Riven mod, [Vigilante Supplies], and [Speed Trigger], and you have an un forma'd build. This 1 build with only 6 mods is 1 shotting level 165 Eximus Corrupted Heavy Gunners and Bombards. How is it doing this?

[Internal Bleeding] procs a slash status effect whenever you cause an impact proc. Coincidentally, the alternate fire guarentees and impact proc on every shot, and even with fire rate mods you're still well below the 2.5 fire rate for the alt fire. This high damage multi hitting shot while still being mitigated by armor, doesn't reduce the true damage slash procs scaled from these high damage blasts. After a couple tics, they're dead.

So that's the 3 crit mods, your typical Seration and Multishot added on top for even more damage, and [Internal Bleeding] core. What else to put on?
[Vigilante Supplies] I would recommend. Each alternate fire spends 50 ammo to use (and force a reload if you don't have it, or just not fire), so you might find yourself running out over extended sprees. I know I did a couple times on extended Steel Path missions.
Fire Rate I've also found useful, both in having a nice primary fire and speeding up the charge of your big shot. However I can't recommend [Vile Acceleration], and [Primed Shred] would be too expensive for an already heavily forma'd build you're seeing (unless you're replacing the Riven with it).
The last mod would be good to put on Toxin damage. Use [Malignant Force] as your primary fire has good status chance to prime them for even quicker killing.
If none of these are up your alley, perhaps a comforting reload speed mod if you don't like its 4 second reload (you learn to get used to it), or maybe even [Bladed Rounds] because you're going to be seeing nice Red Crits and this will out pace [Heavy Caliber] easily without the drawback.
As for the arcanes: Deadhead isn't too difficult to get its 3 stacks and is the most damaging of the 3, while Merciless still racks up with slash procs with some nice reload speed, but it does decay quicker. Dexterity if you want the melee support, but the gun often carries itself well making this choice pointless in my opinion.

The final note I will add is don't think about giving this to a crew mate. They don't like using the alternate fire, they can't aim for the Scope mod, and for some reason they don't get riven stats if used by the "On Call Crew."