The Tch Priest I'm currently playing has Two-Weapon Wielder, Ambidextrous and a ballistic mechadendrite. Without the mechadendrite he could fire both his bolt pistols on full auto as a full action (with a -10 penalty). Can he also fire the mechadendrite's pistol? And at what cost/penalty?

If memory serves, using a Mechadendrite of any kind uses up the character's Reaction in a turn, so they won't be able to dodge/parry without Talents that let them dodge/parry twice in a turn

Actually, only the ballistic mechadendrite's text says it can be fired as a reaction, and the errata changed that to either a reaction or a half action attack, the other mechadendrites makes no mention as to which type of action is required to use them.

As for the original question I would say that you cannot include your ballistic mechadendrite in a multiple attack action, but you can immediately after firing both your bolt pistols as a full action, fire your ballistic mechadendrite by spending your reaction, and that attack would not suffer the penalties the bolt pistols did.

To words... Kaboom! pew!

I'd consider letting the character use his ballistic mechadendrite just like a normal weapon, meaning it can be fired together with another weapon as a two-weapon-attack or as a half action alone. Other mechadendrites and similar implants (helot augmetics) have to be used the same way, with the ballistic one having the advantage of being useable as a third attack (via using a reaction).

voidstate said:

By full auto you mean semi-auto right? Bolt pistols don't have a full auto mode.

And to answer your other question, I would say, like others, that he could use a reaction to fire the mechadendrite. After all, the point of a mechadendrite is to add functionality, and I just don't see it not shooting if it's there. That said, I would probably impose a -10 penalty on the attack for balance reasons.