Bob,
As I recall, it was 50/50 solder. The Monoblock/barrel solder joint was large enough that it was plenty strong enough to hold them together. My experience with this was in the mid 1970s to early 1980s. I highly suspect that now they would use a high strength glue such as Loc-Tite bearing fixer (I think it is their #662 or 668). I recently lost a friend that converted double shotguns to double rifles, using this glue. Due to a mistake, he had the need to break the bond of this glue and he had to heat the barrels/block to bright red to do it. With regard to machinery, you would be surprised how little was required. My German gunsmith friends used only a small "speed lathe" that had a shop-made bed extension when replacing barrels using the old ones to make the "block" ( hakenstuck?). This lathe had no carriage or lead screw. This was before reunification and if they made a new barrel set for an East German gun that no blank sets were available for (Merkel, etc.) The used a small horizonal spindle mill that had a vertical spindle attachment installed. The main "tool" was their experience and skill. To put the parts( monoblock/barrel tubes, ribs) together, they sent them to the barrelmaker( rohermaker?) who had the appropriate equipment (oven). Even though my friend was qualified to fit the new barrels up, he usually called another friend in to do it. This fitter used mostly files and scrapers to fit the barrels in. He filed like a machine and worked right on top of thousands of dollars' worth of engraving without "missing a lick". His main equipment was also his skill.
If you take the project on and mess it up so badly that you can't sell the parts for scrap iron, you still would not have lost enough to outweigh the fun and experience you would have gained.
Mike
To be clear: When you say mono block you are referring to a part that completely encircles part of the chambers of the barrels right? If so, depending on the shape of the barrels mechanical wedging might be taking all the force, and the solder just keeps them from falling out if you bump the barrels with the action open. Sure there is more surface area, but a tapered fit is incredibly strong.
I might own a file or 40, but I try very hard not to need them. That being said I use them at the lathe every day. Well everyday that I use one of the lathes. I hate file work. I recall my dad making me try to file hardened steel a gazillion years ago. The parts I'd make for this would likely be 4140HT (because I have it on hand), and while it is fileable its not fun if you have to remove much.
