I thought that the turrets would be easy because of their size: think again.
First, the barrels. After trial and error, I came up with a satisfactory way of forming the barrels.
1 shows the barrel as cut from the card. Then, the barrel was rolled loosely around a 2mm skewer or dowel (2 & 3). It was then rolled by hand like a cigarette until the grey overlapped the glue edge and then opened slightly until you can see the glue edge (4). The first stage of the barrel was glued by tacking the end with a drop of PVA (see below). The rest of this portion of barrel was glued (5).
The next section of barrel was glued in the same way, tacking it where shown in red.
The rest of this portion of barrel was glued, as shown in red.
Finally, the last stage of the barrel was glued as shown.
The barrel was rolled and the excess glue squeezed out and caught on the index finger.
Barrel mounts.
The barrel mount was reinforced with some balsa as shown, to provide a more rigid mounting for the barrel locating pin which goes through it.
The barrels were reinforced with toothpicks. PVA was squeezed in each end of the barrel and a toothpick “screwed” in from the rear. Some toilet paper was twisted up and used to caulk the base.
This was coated with PVA and, when dry, a 2mm hole was drilled in the barrel mounting block and the barrel glued in and the excess toothpick cut off at the rear of the block.