Hi, Addy, My favorite way to solder chain into a tube to finish it
off (which I do mostly with snake chain) is to insert the chain,
allowing a bit of it to stick out the other side. I then solder it
at that end, and trim/file off the excess. I lay a razor blade over
the other end of the tube and chain to avoid having solder travel
there or melting something. With a closed cap, I would put a piece of
solder in first, then the chain, proceed as above, though of course
there is no way to really know when you’ve done it other than
quenching and pulling. Maybe someone else will give us both a tip on
that. HTH! Noel