Found that you can apply poison to the Handgonne shots, which immediately evaporates if you attack a blob of 4 targets, but I’m not sure mechanics like these were intended since poison is fairly easy to come across (goblin corpses, crafting, alchemist).
I haven’t tried Fast Adaptation with Handgonnes either, but I wouldn’t be surprised if you racked up Adaptation stacks quickly if you happen to miss an entire blob of 6 targets.
It’s working as intended. Fast Adaptation works like it does with any other AoE attack; you can both gain and consume a stack once with a single attack.