Hi Maarten,
In the early days when I thought of RC'ing it my thoughts were tending towards drilling though the inner drum so I could have a cranked link from the outer drums. I was going to use 2 linked servos in the foot well connected to these cranked arms to give the elevation. I would then hide as much as I could with a torso figure.
The top half is seperate from the lower drums and sits into the top ring, so rotation should be straight forward. Might be worth putting a thin nylon ring between the two to act as a bearing surface.
But I'm going to leave you and the others to figure it.......
I am clearly insane as I am already looking at new ammo trays, for traditional ammo boxes as often seen in Nam, and possibly even a limited edition display base in the form of the M20 trailer up on its jacks and without the wheels to show mine off.
One things certain. After this I am not building anything this large or complex again as a commercial item.
Best regards
Dave