Daryl,
Close fitting is very important but on joints with the areas of monoblocked barrels you will not get a one mill fit, nore is that necessary. I try to get 3 to 5 thou. close fit and that is plenty good enough. More important is that you get them properly soldered with as close to 100% contact as you can get. This comes from cleanliness of the metal surfaces, thoroughly tinning the parts, washing off all the acid flux, refluxing with resin flux, and bumping down the barrels to get them fitted as tightly as possible. Bumping down consists of very slightly undercutting the shoulders on the barrel stubs, and the front surfaces of the monoblock. When these two surfaces are driven down with a brass hammer, the undercut surfaces slightly mash together to make a virtually invisable joint.

This is how the British used to do their shotgun monoblocking. A hardwood block is set on the floor with the barreled monoblock rested on it, then after reheating, the barrels are "Bumped Down", their words, not mine. This is done with a fairly heavy brass or copper hammer. A steel hammer can be used if you put a heavy piece of maple or oak wood over the muzzles, but if it breaks during the process, you could easily damage a barrel or two. You can't do this procedure with threaded barrels.

On the first gun I built, I didn't have a lathe and I hired a guy to machine the barrel stubs and thread both the monoblock and the barrels. I told him how I wanted it set up so the each barrel would be properly lined up with the monoblock. Long story short, he didn't listen to me and screwed up the threading something horrible. The barrels not only spread as the went towards the muzzles, but they would not seat down correctly against the monoblock. I was finally able to get it finished but not without a bunch of trouble. I have since bought my lathes and have not had a repeat of this mess.

Whenever I can, I bore out the monoblock to completely bore out the old rim cuts left over from the shotgun chambers. This leaves very little wall thickness on the front of the monoblock but that really doesn't matter, the barrels are fitted up against the standing breech face so that they don't impart any recoil pressure against the joints anyway. This method gives a very tight joint visable from the breech end and none of those silly rings to fit up and solder in.

This fit up will not work on some lighter weight gun frames because the barrels reduce too quickly in diameter to allow for a decent length monoblock. Don't try this with lightweight bird guns, you want a standard weight donor gun to start with. Because of the amount of the areas on the barrel stub joints, a very thin monoblock front wall can be used, this also leaves the barrel walls thicker in the chamber areas which is very important. None of this will work with threaded barrels. Bob