With the armor Changes now this is the most fun beam you can get: Just hold down Mouse 1 and everything goes boom every few milliseconds.

Riven Guide: you want Firerate and Multishot and the 3rd is flexible for either an element: to swap out [Atomic Fallout] and create corrosive as well but heat or cold would probably be better or crit chance and the slot [Primed Ravage] instead of [Atomic Fallout]. This requires another forma on [Chilling Reload].
for a rivenless build you would slot [Shotgun Barrage] instead of the Riven.

The Empty Slot is for [Shotgun Elementalist] to add another Multiplier to your Blast Explosions.