Waidmannsheil,
I understand your idea about doing the threading precisely but to compare the threaded barrels on bolt actions and the threads cut on the inside of mono blocks is like comparing apples to oranges. The bolt guns have very thick walls and they are screwed into thick walled actions. With a monoblock you don't have the luxury of all that barrel thickness, no matter how fine a thread you cut inside a monoblock, you will substancially weaken the walls of the monoblock up to or exceeding 50 percent. It can't be helped because you are cutting away half of the wall thickness of the monoblock.
By using Hi Force and getting all the mating surfaces thoroughly tinned before trying to put them together, you will get almost 100 percent of the holding strength of the solder, which is listed at 28,000 PSI. If we round off the circumference of a typical barrel stub as being 3 inches around, which is pretty close in most cases, then take that times the length or the stubs, another three inches, that squares out at 9 square inches of mating surfaces per side.
When you multipy that out you will come up with 252,000 pounds of holding strength PER SIDE. The British and the Italians have been monoblocking barrels for many years and using softer solders than this to do it. This is not theory, I have done lots of guns this way and none of them have come apart yet. Bob