The unique effect of the [Sagek Prime] makes it super easy to get 10 stacks of radiation if you only mod for that. Secondaries getting two primed elemental mods provides you with the highest possible radiation combination. The rest is focused on decent crit, high multishot, and fire rate to trigger the Secondary Irradiate effects as soon as possible. Merciless Gunfight is very good because punch through will let you proc radiation on multiple targets.

Pairs even better with [Nyx] and her Psychic Bolts augment to spread radiation while armor stealing and increasing your crit chance even further.