Ron said: "Because the .410 action is so small, and the tolerances so close, I don’t feel comfortable to thread the barrels and the mono-block, as I have always done in the past. I therefore decided to shrink-fit the barrels into the mono-block by shrinking them with liquid nitrogen"
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You certainly are not afraid of innovation. I would have simply soldered them in place, while keeping the solder away from the seam area. Soldering them alone; not threading, works because the soldered surface is so large. I have used half threads/half soldered, and fully soldered; not threaded at all, both systems worked. Now, I am anxious to see how liquid nitrogen tight shrink fitting works.