The thermal cycling is no problem. Salt baths can be used for that to great effect. I had already planed on refining the grain as I don't think the barrel would survive heat treating without it. I used to forged japanese style swords, and still have long tanks and am used to heating and working 30 to 40" bars.

A layer of tin around the mandrel should help remove the mandrel after winding the riband. I wonder if stainless heat treating foil would do the same....