Yes! You are absolutely right regarding that there are a whole load of different soft solders on the market.
Just to clarify my point I refer to good quality soft solders like the one mentioned by Marraki - not the stuff you might use for electronics. (In my world "soft solder" is good quality stuff that still don't need the metal to be heated to temperatures as high as needed for ordinary stiff stick silver/cadmium solders. Sorry, should have been clearer on that!)
I am also very sure about good quality soft solders and their ability to fix barrels to a mono block without the use of threads. This not from own experience but from what friends has done.
Now you guys might wonder "what kind of friends does this silly bugger keep?"

Well, one of them has built a few doubles, others just one. Place I met them all was at the Swedish school of gunsmithing, and one of the friends I keep from back then is the former head master, his name is Stanley Stoltz. He also happens to be one of the better engravers around, so I will try to paste a few pictures with this post. (After trying to do just that I realized I need to link to a separate web page with pics, right? Gosh, I don't have a home page so posting pics will be difficult for me. Is there any way to post pictures on here without having to link to another page? Help needed from you guys!)
Anyway, he and the other teacher taught us the "ream out and solder" method when it comes to fixing barrels to a mono block.
But then again I was a bit unclear about explaining this (I didn't even mention it!) so I will correct this by saying that this method does only apply to lower pressure cardridges - as the mentioned 8x57JRS for example. I am not familiar with the actual shear force a good soft solder withstands, but the actual surface holding a barrel in place would be something like this (just an example).
Imagine we are using a 12 bore for building a double (there are a few important drawbacks about using a 12 bore for building a double but I won't list them all here except for saying it has, amongst other things also to do with leverage).
Once the chambers has been cleared for soldering (reamed out, barrels shortened to... let's say 75mm, and yes I will stick to metrics here cos I am no good at maths using inches) they will be roughly 20mm in diameter. That gives a total inner surface of (20 x pi (3,14) x 75) = 4710 square millimetres, or 47,1 square cm's
Now a good solder will hold a lot covering that much surface. Just HOW much I don't know...!

But as I said b4, none of them doubles made has turned to scrap - nor their owners.

And here is an open invitation to anybody who knows anything about how to calculate this stuff when it comes to shearing forces neccesary to pull the barrel out of place! If anybody knows how to calculate this then please show me how to, please! I am very sincere on this!

Regarding my own project I am turning towards the 45-110 ,the 45-120 or something similar. As the action I am going to build my gun on is old (90+ years) I will use nitro-for-black loads only. I happen to own a 12ga gun (see my presentation under "general" on this board) equipped with an extra set of .450 bpe barrels from the manufacturer (Husqvarna) so the action itself does allow for BP pressures. That 12ga will be for sale shortly, but that is another story...
(Edited: Forgot to mention here that I will be using a 16ga action for my own project, and that YES - I am in such a lucky position that I do have good friends at hand that knows the process on "how to" and "what not to" do. If you reading this don't - then by all means DON'T DO IT!)

The problem about having the gun proofed will be dealt by having it sent to Norma cartridge company. I still expect it to return in one piece, but I want somebody knowledgeable about proof loads doing the actual testing - I am not going to fill a case at random using bullseye and "see what happens"!

Any thoughts on this much welcome!

Per