Hi! It's great to see your studies up, they're really great O:
The only recommendation I have is to put some a greater variety of value in your pieces. Lighter lights and darker darks packs a punch and makes everything look much more three dimensional.
I took the cannon you did and put it into greyscale to show you what I mean. For a paint over I just took a soft brush and a black and just lightly brushed over it. Then I took a very light grey and went over the highlight.
The same could be done for the wood it is sitting on and the other pieces. Don't be afraid of going more extreme with your values! Metal, especially, because it is such a reflective material.