You have two sizes of tubing that could possibly telescope but the inner one is slightly larger and won’t fit into the outer tubing. You need to stretch the outer tubing.
Cut both pieces of tubing to desired size. The outer tubing should be cut a bit longer since it will shorten once you stretch it. That metal has to go somewhere!
Anneal Outer tubing.
Use small Mandrel (this could be a drift pin, a scribe, anything tapered that fits the size you need). Slide outer tube onto mandrel and mark how far it goes on the mandrel with a sharpie.
Line up on a bench vise and close it but allow room for movement. Hit mandrel with a mallet making the tubing bigger. Mark with a sharpie how far it is on the mandrel.
Flip the tubing over and do the same up to the line. Check if your inner tubing fits. Is it slightly too large? Place your outer tubing on the mandrel again and tap a bit more being careful not to go too far. Remember to tap the other side. NOTE: you might need to anneal depending upon how much you have already stretched.
Check your fit, the inner tubing stays in but doesn’t slide in easily, perfect.
Now take a bench block, steel part of pliers, something to give you leverage and push the inner tubing into the outer until the bottom of both pieces of tubing are flush.
Solder together, pickle. Your setting is ready for use.
NOTE: you may have to use a round or setting burr to fit your stone properly.