This is my [Arca Plasmor] build for the Index. Obviously this build can be improved on by, for example, maxing out the [Primed Point Blank] so it can provide a 165% damage boost before the [Blaze] mod comes in, replacing the [Charged Shell] and [Cleanse Corpus] with their Primed variants, and adding in a fire rate mod like (Amalgam) Shotgun Spazz, [Frail Momentum] (never ever go for this, only villains do that) and [Accelerated Blast] (there's no point because [Arca Plasmor] deals pure Radiation damage and it will never deal Puncture damage). However adding in the suggested mods will require forma;

Having [Primed Point Blank], [Blaze], [Charged Shell], [Chilling Reload], [Contagious Spread], [Hell's Chamber], [Cleanse Corpus] and Shotgun Spazz will make you go over the limit thus you will need to add a dash polarity where [Charged Shell] is and a v polarity where the Cleanse mod is, resulting in

TOTAL DAMAGE: 22,308.0
AVERAGE HIT: 25,252.7
BURST DPS: 52,778.1
SUSTAINED DPS: 37,220.1

Having all of the Primed mods will require another v polarity resulting in

TOTAL DAMAGE: 31,585.1
AVERAGE HIT: 35,754.4
BURST DPS: 74,726.6
SUSTAINED DPS: 52,698.6