Attached the recoil tube yesterday. But I cheated. It is a gas strut.
But, it is SO close to the dimensions that I required, that I decided.. what the hell. It is 18mm diameter (17.5mm required), and 200mm long (198mm required).
I degassed the strut by drilling a 1mm hole, and the gas came out under considerable pressure. I had to do that, because the strut was too strong for the cannon. Even degassed, the strut has enough shock absorbing action to be useful and realistic. I made some brass brackets and a cap, for visual consistency.
Today I made the lever which locks the elevation gears.