When mounting round stones of 5.0mm or more it is time to give up tubes and use strip. But not the stuff you buy which is flimsy and usually much too wide. Join the club and cut it from a sheet of fine silver, say 0.5 mm thick. The width needed for most cabochons is 1.5 to 2.5 mm, just enough to engage the curved part of the stone.
If you can find a cheap set of twist drills, going up in half sizes to 10 or 12 mm, their shanks will provide an ideal set of mandrels for making bezels (and incidentally jump rings). Measure your stone with callipers and wind your strip around the relevant drill shank or one slightly smaller. Once cut and soldered its size can gradually be increased by either forcing it up a tapered triblet, with your fingertips, or by light taps all round with a hammer. The fit should be snug but the stone should still pull out with Blu Tack or a wax cone. Shallow cabochons can be made more prominent by adding a jump ring or washer to raise them, but this does not necessarily have to be soldered in
For faceted stones the bezel will be the same depth as the stone, measured from table to culet, but you also need a bearer. This can be a second tube fitting closely inside and soldered to support the stone at the correct height for setting. A more professional option is to raise some spurs inside the first tube, using a scorper, to support a washer or jump ring, which is then soldered in place.
For a splendid finish it is also worth considering setting from behind. Make one tube so that it will admit the stone and add a narrow domed washer to the front. Adjust the window so that it will only just retain the stone, and polish. Push the stone in from behind and then add an inner tube with its back end made flush and slightly bevelled. Then close the back of the outer tube around the bevel.
These are quick fixes, but I do think that faceted stones are more elegantly fitted into tapered collets. This does away with the need for bearers, although setters often cut a slight ledge with a burr or graver to steady the stone. For small stones you can use tubing and coax it into shape in a collet block, but for larger sizes I prefer to make a cone shape out of sheet first and then perfect it in the block. Dennis.
Below: My box of drills.
Bezel made from sheet.
Set from behind.
Cone shaped collet.