During this stage I went back and attempted to remake my wood grain in Zbrush, bumping down the detail from last time. Unfortunately I wasn't able to get to and detail every piece, so some have some blank spots that are missing detail. I did have a minor issue when baking involving the high poly, that being that there is a spot on the wheels where backfaces clip through, and this showed up on the AO map in my bake.
Here are my renders in Zbrush for this stage:



The low poly version of the cannon is 5,058 triangles, here is how the wireframe looks:

Here is my UV map, I didn't change it from the high res/UV stage as it worked, however some of it does need some adjusting in terms of orienting shells and laying them out:

Finally, this is how the cannon bake looks in Substance and Unreal:


Unreal shot with mid-gray material:

Unreal shot with basic textures: