Taylor 416,
The rear of the barrels are cut off at 2 1/2" to 3 1/2 "long to form the monoblock, then the chambers are bored out to give it straight parallel sided walls. The barrels are then turned to the proper profile including shouldered stubs at the rear that closely match the bores of the monoblock. The mating parts are then tinned with solder and installed. Some people turn partial threads on these parts but I have found that not to be necessary if you get a complete adhesion of all the parts with the solder.
I have developed special techniques that allow me to do that and get a 100% fit and fill of the solder into the joined areas. I use Brownells Hi-Force 44 solder which has a holding strength of up to 28,000 pounds at 100% mating. Each barrel stub has appoximately nine square inches of area to be soldered so that equals 252,000 pounds of holding strength per side. I also use it to install all my spacers and the ribs which are completely soldered to the barrels, this makes a sandwitched and reinforced unit out of the barrel assembly which is hugely strong.
Many guys use soft solder on the ribs because it is easier to do, but I feel they are missing the boat, so to speak, when it comes to making a strong barrel set. Using the Hi-Force 44 through out the whole process makes the barrel set much stronger and better able to resist the forces trying to seperate them. Because the ribs are also firmly attached to the barrels, the contures can be made somewhat lighter because each barrel and the attached ribs are helping to support the other barrel during the firing of the gun. Hi-Force 44 costs more and is somewhat more difficult to clean up, but it is worth it to me because of it's extra strength. Bob