Oh, this one took off!
Being away from the net for a few days and I see that there has been some discussions going on. Oh well, I believe that some good has come out from all this!

Yes Unspellabe, you got a point. Shear force is what counts when it comes to holding the barrels in a mono block. Let us now (just for the FUN of it - I won't be around til monday to "defend" my opinions cos I am going away for something that we up here in the cold north refer to as "midsommar" - midsummer. Yep, we actually still celebrate the sun up here in the sense that we have kept up the tradition of partying our socks off on the longest day of the year! ) take a look at what kind of forces that affect the barrel-to-mono block-joint.
Assuming we are using a cartridge generating a chamber pressure of 25.000 psi (I am in way ower my head here cos I really should be making these calculations using the metric system. Oh well..)
Then we get a chamber pressure of exactly that - 25.000 psi. Yep, Pounds per Square Inch.
Now, how large an area does this theoretical cartridge have on it's base? Again assuming that it has an diameter of ½", it would be (and now my maths is failing me but isn't circular areas calculated by taking multiplying PI (3.14) with the (radius square 2)
But to make it simpler, let us say that the base area of the case used is 1/4" square.
Now that means that the shearing force the solder joint would need to have not to break under the pressure from the shot fired would be 1/4 of chamber pressure (this not taking into account that the case itself does cling a tiny bit to the walls of the chamber. However, a brass case won't stand to all that much pressure so we will just forget about that to keep things simple).
1/4 of chamber pressure would in this case be (25.000 divided by 4) = 6.250
One square inch of relatively soft solder will hold about 4.000 pounds before it shears - see link;

http://www.efdsolder.com/prod_alloys.htm

Sorry, but melting temps are graded using Centigrade.

If we then assume that the area holding the barrel joint is equal to 6 square inches (see earlier post that I made, I don't wanna take you through that calculation again except for saying that one square inch is equal to 6,45 square centimeters) the joint will hold 4.000 x 6 = 24.000 pounds.
Now that is pretty much to hold down a thrust force of 6.250 pounds, isn't it?

And now - PLEASE correct me if I made any faults in my calculations!

When it comes to choosing the "right" size of shotgun for building a rifle I would say that the smaller you can get your hands on, the better. A 24ga would propably be great! Husqvarna made a series of double rifles in 9,3x74R on their model 310 (anson deeley type) 20ga action. I even aquired one of those rare shotguns (about only 500 20ga model 310 were made) just to build my own set of barrels. But then again I sold that one off in about 2 days time... Easiest sell I will make. Ever! Quite profitable too...

The good part about using a smaller shotgun than 12ga for building a double rifle is that the distance between the center of the bores versus the action itself gets smaller. Hence less torque on the barrel to action joint.
Really neat doubles are built on small actions. So a good start would be 16ga or less. I have heard that at least one of Husqvarnas double rifles were made on the 310 in 12ga - still chambered for the 9,3x74R.
Just remember NOT to use any old shotgun - go for a good solid quality one - and have it proofed!

A relatively cheap action that still is somewhat available is the Merkel model 8 in 20ga. A good starting point! If lucky you can pick one up at about $800 US. At least here in Sweden, that is.

When it comes to barrel diameters necessary I am not the man. All I know is that the outside diameter of the rifle barrel on my Merkel SLII combo gun in 20ga/7x57R is relatively slim. But as I don't want to run off to the gun safe and take an exact measurement right now I'll post the exact O/D of the chamber only if somebody asks. But take a look at any drilling or combination gun, and you will see that the diameter of the rifle barrel thins out rapidly. So there is no need for THAT much material to keep the demons of pressure at bay.

To sum his my post up I would like to add that whatever you do - IF you build yourself a double rifle - make pretty darn sure that you have somebody at hand that can show you hands on how to, and that you get it proofed before you actually fire it from the soulder!

Kind regards

Per