Had this come up our last game. The Ballistic Mechadendrite (pg. 190) states that "the heretic may attack with this weapon as his Reaction." When exactly does this happen? A few possible interpretations:
1) You attack with it on your turn, using up your possible reaction in order to do so.
2) You attack outside your turn at any point, sacrificing your reaction.
3) You attack when a reaction is triggered on you, such as firing the mechadendrite instead of using a dodge or parry when attacked.