Still - you can use the M2 against the Chieftain in the Structure - but since he is not a structure or vehicle you wouldn't be getting the +5 to kill/pen.
You sure the Chieftain is not a structure ? he is an objective and the objective got the keyword "structure".
In fact that is the whole point of my enquiry.
The chieftain being cover 6, seems to indicate he is in fact inside the structure.