George, why do you think there needs to be compression on the breech band? I would agree with you a little if this was cast iron or bronze. But in this application you only need something to retain the barrel in the sleeve. Neither N-SSA and AAA call for shrink fitting liners. Switlik even describes the process of slip fitting the liner as the best method of fitting the liner. After all that's all this barrel is is a liner the that sticks out the end of the sleeve.
When lining a bronze, or especially a cast iron, barrel, we want the liner to take the majority of the stress/expansion and by leaving a space or a resilient filler between the liner and barrel, that is what we get. When putting an external reinforcement over a chamber, we want the reinforcement to be engaged from the beginning of the expansion, not only after the internal tube has expanded to fit the hole in the external tube. Otherwise we don't really have a chamber wall that is chamber diameter in thickness.
I suppose that making the diameters exactly the same would be close enough and that would be easier to push the one onto the other, but the shrink fit guarantees the coupling between internal and external almost as well as if they were one piece. The retaining of the sleeve is only inertia versus recoil; the shrink fit is more than sufficient for that. With a tight slip fit, I would probably lightly weld the backs of the two pieces together. On the other hand, since you probably have machined the outside of the 2.25" tubing to get a true cylinder for the reinforcing sleeve, the shoulder at the front of this will keep the sleeve from moving forward, which is the direction it will want to move from firing. And the vent liner will also resist movement.
