For Casual Steel Path, just replace Primed Expel Grinner with Fulmination and you'll be good :D
Evolutions:
1: Headshots Charge Incarnon mode. To aim heads better, I am using the Heat stun, Steady Hands, and Deadhead for zero recoil.
2: Rapid Wrath: +20% Fire Rate, stacks with Lethal Torrent into 80% Fire Rate.
3: Lethal Rearment: On Headshoot, it provides +30% Reload Speed for 12 seconds up to 3 times, that is 90% reload speed in total. This is a Primed Quickdraw at hand, just aim heads.
4: Elemental Excess: +20 status chance and -10% Critical chance for tier 5 evolution.
5: Devouring Attrition: 50% chance to deal 2000% damage on non-critical hits. This damage is multiplicative to raw damage mods such as [Hornet Strike] and elemental mods.
Disclaimer: If you don't have Nourish for Viral then Roar, Eclipse, [Xata]'s Whisper, or another damage buff would work very well too. Overframe thinks that this build is 1.4 million DPS with Cascadia Flare but you might be better off with Deadhead for a higher multiplicative headshot damage so long as you can aim heads.
Even if this build wasn't intended for general use, it remains very powerful even if you don't have either Nourish or [Xata]'s Whisper. Enjoy :)