There was a "contest" at the old cardmodels.net that sounded "How small can you roll" And as far as for tubes....you can make pretty small ones with some practice.Tubes wit a diameter of 1mm is not unusual.Barrels for machinguns f.eks.

But when making small tubes the same rules goes.Use a tool to shape it well before you glue it together.
This example is of one of the axels for the rear wheels on the Luchs.
The diameter on the tube is 4,5mm
First I cut away the flip at the end


And glue it inside as shown before


Next is to find a rod with a proper diameter.The rod I used was a 1,5mm welding electrode.


Now after some rolling the tube is tight enough


Then it's to glue it together applying some pressure at the joining point with some tweezers and we have a nice round tube.


More to come.....