I wouldn't make it one shot. I also disagree with you even thinking of the chariot rule of autoexploding. I would give it impact hits. Either d3 or d6, but not too many attacks (4?).
A cannon could work, though what you've drawn seems more like a camera

Having a pilot at the top reminds me so much of Labyrinth. I loved that clockwork giant with the crazy goblin in it.
Am I the only one looking at your gobbo/dwarf threads?
