PART 1: https://www.halloweenforum.com/threads/how-to-make-a-realistic-cannon-part-1.79204/
PART 2: https://www.halloweenforum.com/threads/how-to-make-a-realistic-cannon-part-2.79205/
PART 3: https://www.halloweenforum.com/threads/how-to-make-a-realistic-cannon-part-3.79207/
PART 4: https://www.halloweenforum.com/threads/how-to-make-a-realistic-cannon-part-4.79208
Here with go with more details on the "Geoff design" cannon:
You already have the tapering cardboard tube and the muzzle section done. The rear rounded part of the cannon is simply a plastic bowl! I managed to find one in the dollar store that was the exact size the the cannon at the back. It even had a flat bottom that allowed a plastic ball to epoxied onto it.
I wanted a "steel band" around the back of the cannon between the bowl and the tube. This was just another disc of pink foam that I sanded to be more rounded on the edges.
The "stanchions" or posts that come out of the side of the cannon were sections of ABS pipe. I carefully cut a hole in the cardboard tube on both sides and slid the pipe through. Note: if your ABS muzzle tube goes down the cardboard tube too far, you won't be able to have the stanchions go all the way through the body of the cannon. In my opinion, it's better to make the muzzle tube a bit short so the stanchions go right through the cardboard tube are good and sturdy.
The "reinforcing bands" going around the cannon are just strips of heavy cardboard (I used another builder's tube for strips) glued into place. To give it some extra shape and dimension, I even wrapped some plastic piping around the center of each band, and then blended it in with latex caulking.
Caulking. Caulking. Great stuff. Use it wherever there's gaps and cracks. Who cares what it looks like unpainted because the next step is...
You guessed it...