I contacted either a Spanish or Italian maker years ago about getting some chopper lump barrel blanks. They wanted $750. I bought Galazan tubes for 200.

BTW,
Galazan uses this method of joining the tubes. Note the register pin hole. The assembly is silver brazed at the breach end. What alloy they use, I don't know. As you probably know, there are many alloys with melting temps all over the place.