Oncall crew take no conditional mods or arcanes, so [Galvanized Chamber] will not work on it. Primary Merciless is used purely for the reload speed.

We use [Primed Magazine Warp] so our oncall can shoot 3 more Zarr rounds before reloading. Vile Acceleration is used so our On-Call fires rounds faster leading to more DPS/more kills. Primed Firestorm is so the aoe can hit more enemies.

We can go toxin progenitor so our oncall can slot in [Primed Cryo Rounds] and do viral damage and proc viral status for the viral status damage boost. If you want corrosive, switch to [Stormbringer].

Another alternative is to go magnetic prognetor and do corrosive cold so it also does more damage to overguard and shields. You will replace [Primed Magazine Warp] and [Vile Acceleration] with [Infected Clip] and [Stormbringer].