This build for the [Rauta] is primarily designed to increase its combat efficiency, as well as reliably trigger Slash and Viral procs.

Faction Damage mods aren't really negotiable; with [Rauta]'s low base Crit Chance, you're not going to spike damage for powerful bleeds as well as hybrid weapons, so you'll have to rely on Slash procs double-dipping on Faction Damage multipliers to get the bulk of your damage.

Other Notes

• [Sweeping Serration] is used to more heavily weight the proc chance per status towards Slash, since that's going to be the bulk of this weapon's damage.

• Shotgun Barrage can be replaced with its Amalgam counterpart to no detriment, if team utility is desired.

• [Seeking Fury] is a flex slot; it can be replaced with any desired mod. Worth considering [Hunter Track] as a replacement to prolong the bleeds and provide synergy with companions. Alternatively, Shotgun Elementalist can increase the damage of your bleeds while providing increased magazine capacity to work with.

• Primary Dexterity is technically replaceable, however the Rauta's unique gimmick of increasing your Melee Combo Counter on-hit really encourages the use of melee weapons, and in that regard, Primary Dexterity's +7.5s Combo Duration is very welcome.

Use Cases

The [Rauta] exists in an interesting space due to how it encourages the use of melee weapons, which gives it a pretty broad spate of use cases where running it would be desirable;

• Melees with low base attack speed can have their combos stacked up faster, which can kickstart Berserker Fury where applicable.

• Heavy Attack focused builds can now get more convenient damage spikes without having to dawdle with weak light attacks or fuss around with bonus Combo Count mods.

• Warframe abilities that scale off of equipped melees and combo count ([Khora]'s Whipclaw, [Atlas]' Landslide, [Gara]'s Shattered Lash, etc.) now can deal consistent damage without having to waste slots on the weapon for Combo Duration, and it opens up those builds to running Focus schools besides Naramon.

• [Ash] in particular can get ample mileage out of the Rauta, as the gun effectively serves as an alternative to his Rising Storm Augment, albeit with 2.5s less combo duration - provided Primary Dexterity is used.

It is worth noting, however, that the [Rauta] does not function properly with the [Venka Prime] at present; resetting the combo down to 12x even with [Venka Prime]'s 13x Max Combo Multiplier.

EDIT: As of update 35.1.0, the [Rauta] now properly stacks [Venka Prime]'s combo count upwards to 13x.