I can't get Gunslinger Challenge kills to count while in Showdown?

I'm using an Explosive Pistol, for Showdown, and trying to use the Auto-Crit-lock-on from Showdown to do the Gunslinger Challenge easier.

The challenge being, "Kill enemies with pistols shortly after looking down the sights."

But I can't get such kills to count while in Showdown? Or am I doing something wrong? I'm looking down the sights for an instant, before firing my bullets...