Unlike other Gun Damage boosts, Reaper only affects certain types of splash damage. Reaper does not affect a gun's splash damage unless the splash does not count as grenade damage and can process Blood Overdrive, but not Blood Bath. Splash damage boosted by Reaper includes:
Maliwan sniper rifles, including the Pimpernel, but excluding Volcano and Storm
Hyperion/Tediore Splatguns, including the Slow Hand, but excluding the Retcher
Reaper affects all enemies who have more than 50% health remaining. This includes enemies with full health but depleted shields. As a result, shielded enemies will take the increased damage until their shields are fully depleted and their health is half depleted.
The damage bonus is multiplicative.
↑[Guide] Splash damage guide - Grenade, Reaper, and Launcher splash