Well I am yet to experiment with controlling other objects with mouse x,y,z commands... but urm in theory you could ray cast mouse z to get a coordinate for the canon to point to perhaps?
But the controlling of the canon hmm... perhaps use a IF PLAYER IN RANGE OF CANON then PRESS ENTER KEY TO CONTROL CANON that would be your initialise script then WHILE PLAYER IN RANGE OF CANON canon AIM MOUSE X,Y,Z...
if you follow me...
I may figure this out in future when I toy with more advanced techniques but for now still going through networking systems... hope this helped
and always happy to help out