You have 25 mm of 5 mm tubing and want it bent into a smooth
curve…very difficult to do. A short piece like that affords no
leverege for bending and as you know, tubing will kink and fold
just like a drinking straw, rather than bend smoothly, the way
you want it.
I have heard of people filling tubing with a variety of things,
sand, clay, ice, etc. but my own efforts in those directions did
not work satisfactorily.
Here is what works: Take a piece of the silver tubing much longer
than you need. Anneal the silver tubing. Go to a hardware store
and find a large spring that fits snugly around it. (Check out
the kind used to close a screen door.) , With the spring around
it, bend the tubing and spring against something like a baseball
bat. If you have enough material you can apply force with
leverege. The spring will keep the tube from collapsing and the
bat will guide the curve. After bending, slip the spring off to
reveal gently and evenly curved tubing.
Revere Academy of Jewelry Arts