What is the upgraded Critical Specialization from the Grievous rune for Firearms?

[Neberu]

Gun's and Gears describes the Critical Specialization for firearms but nowhere that I can find does it say what the upgraded version should be if the firearm has the grievous rune applied to it. Is anyone aware of anywhere a Dev has stated what it should be?

[aobst128]

It's the same effect as the sling group, so I would assume it has the same grievous effect as well.