Hi, FYI, the grid is in inches.
Assault cannon took a little of scratching to make it look better (and adequate to the scale).
Ammo box - added a few parts to recess the "windows", made the shells from a wooden skewer painted golden, and added the links to the belts. Yes, and the bolts or rivets or whatever this is.
The "body" of the cannon - this was a bit of a puzzle to figure out how it goes together. Almost no scoring to make the shape round.
The barrel assembly - rolled paper, rounded segments (I used the sides from the kit, but not the faces - these were really too polygonal). I redid the muzzle area (still not very probable, but looking better than the original "muzzle" in the middle). As a reference I had this similar Dreadnought Assault cannon:
...and ended with this:
Finished. A big gun. Enough said.
All right, we have a problem here - the assembly is pretty heavy, mainly the barrel cluster. I should have put some counter-weight into the ammo box to get it more balanced. Perhaps I will do it yet, but it will take some operating

Scalpel, please...