Updated build for 30.5 -
Since pure damage scaling isn't efficient now with the introduction of secondary arcanes, [Hornet Strike] cane be substituted out in favour of crit (I.e. the addition of primed pistol gambit. With Knells passive, this build will exceed 100% CC - Mainly important for crit consistency).

Alternate substitute mods in place of [Primed Pistol Gambit] can be primed expel {faction} or [Gunslinger]. Both options have minimal damage reduction compared to [Pistol Gambit]. [Primed Target Cracker] surprisingly doesn't compete with the aforementioned mods since the passive doesn't scale off of additive crit damage through mods.

Riven Suggestions
Ideal Knell roll for DPS is +MS +Heat/Tox/Cold +CC -Harmless (Ideally impact). CC is the flex slot, other stats which can occupy its space include +Fire rate +CD +Grineer +Corpus. Ideally your riven will occupy an element mod slot.

At full galvanized/arcane stacks, expect the knell to require 3-4 headshots to kill a 185 heavy gunner. All builds and mods suggested were tested in both simulacrum conditions vs lvl 185 (Grineer Heavys, Corrupted Heavy, Grineer Bombard, Corrupted Bombard) followed by 1Hr SP Mot runs.