Fletchedpair,
You are quite correct. When the sleeving process was in development, the 'inventor' made up a sleeved gun with nothing but a push fit to hold the barrels into their stubs and soldered together at the muzzles. This fired many cartridges without budging at all.
Apparently, the sleeving tubes behave in a similar way to a modern cartridge, they expand into the breech stubs and lock themselves in place during the firing of the cartridge.
I have had numerous guns TIG sleeved and, as with Gunman's comments, can only speak from experience: none have moved, cracked or done anything unexpected. My main personal gun is a sleeved Blanch back action SLE. It was soft solder sleeved (by Westley Richards) and movement was detectable at the breech in the form of beads of oil when the barrels were flexed. However it never moved outwards. I had it TIG sleeved for aesthetic reasons and have never looked back.
