I make a series of pendants that require a ring to frame them. I use 2.5mm 14k tubing that has 27 gauge (.35mm approx) walls. I use 24 inch lengths and coil it to make the rings. I tried a number of ways, fill the tubes with frozen water (I once saw saxophones made this way, I filled them with fine salt, I used an interior wire…
The easiest way I found was to anneal the tube, clamp the wooden dowel I was coiling around vertically in a vise, clamp the end of the tube to the base of the dowel, perpendicular to the dowel using a vise grip locking pliers (you crush about 6mm of the tube), then just coil it as tightly as I can using my fingers. Nothing inside the tube, I get no kinks and the tube stays round. I just need to be sure to use the right diameter dowel so after a little spring back in the coil my rings are the correct diameter. Simple but works like a charm.
I’d suggest practicing with brass tubing of the same size first. Hope this helps somebody.