I have a Canadian friend who has built eight SXS double rifles ranging in caliber from 450 No. 2 Nitro Express down to 22 Hornet. I refer to the Hornet as his stopping rifle for ground squirrels. They were almost all built on CZ shotgun frames using the method cra1948 describes above. You have to really strain your eyes to find the joint between the end of the monoblocks and the beginning of the barrels. They are all works of art that include fitted cases he also made.
He has used them in Africa, Alaska, across the states, and, of course, Canada. My favorite is in 303 British, made with two barrels off the old Ross bolt actions. The thing is quite the shooter and is what he used when hunting deer here with me in AL.
