Proper radiusing of the breech ends and barrels can create an invisible joint without resorting to welding on the barrels.
The French developed the "cannon frete" system, where the monobloc is shrunk onto the barrel to avoid the theoretical problems of the barrel leaving the monobloc. The same system is used in artillery pieces. Practically the problem of the barrel leaving the monobloc seems to be nonexistent. Primitive zip guns prove that barrel and cartridge recoil in one sealed unit backwards. There are some intersting vids of such deviceson Youtube which cast doubt on some old theories re action flexing etc during firing.
There is also the option of threading the barrel into the monobloc. The Baby Bretton has threaded barrels which can be changed by mere hand pressure.
