Birdhuner, thanks for the clear description of the process.

Maybe a note is due regarding "fretting" the monobloc as used by several French makers. It is in essence a hot version of the shrinking-on done by Ron Vella using liquid introgen. Whereas Ron shrunk the barrel, the traditional method expanded the monobloc. In fretting, the monobloc is heated to expand and accept the barrel. The cooling induces a shrink fit, and from what I read this method was copied from the manufacture of artillery pieces.

Then comes the Proof which involves firing two overpressure shells in each barrel. A gunsmith friend muses that the superior proof so liked by the French shotgun makers was employed not only for Proof purposes but also because it provided a final "set" of the barrels in the fretted monobloc.

During a visit to the Beretta factory I saw shotgun barrels waiting for joining. The machined monoblocs were put on one of the barrel stubs, as a way of storing the sets to be joined, and obviously the fit was fairly loose. Obviously there are differences between shotgun and rifle specs, but still, the apparently slack fit was surprising.