If I use a mechadendrite as a weapon, what exact skill do I roll on? If I use a utility mechadendrite as a melee weapon do I use WS?

Why for the heavens am I doing this?

Ok. It depends on dendrite. BS for Ballistic mechandendrite ("Designed for ranged combat, this mechadendrite is fitted with a weapon that functions as a laspistol"), WS for Manipulator, Medicae and Utility ("It counts as a melee weapon"). Pages 183-184 of Corebook, respectively.

thanks. I really need new glasses....