You could have spent a lil more time on the ammo feed =P
Well, all the comments i can think of are about how the cannon needs some sort of aiming and sensory equipment (it would look sweet with some glowing lenses and watnot), and about detailing (some areas are very detailed, others have almost no detail) and layout (why is there an ammo box on top??).
Looks cool, but not practical as a cannon. Pretty nice.