Ken,
as to the depth of the joint....it goes all the way through the mono block. Sleever tubes are usually turned to the rim diameter. Ellis Brown's (shameless plug for a friend) book on building double rifles on shotgun actions outlines the basics. He threads his tubes for rifle barrels but this is not need for shotgun sleeving.

Vintage barrels were not heat treated so heating them to critical temperature or beyond will not have an effect on them. You do need to be careful with modern alloys. I hate heating up large masses of steel so I would likely look at setting them up in mill and cutting them apart with a slitting saw. It would require careful set up but that is just part of shop life.

Keith Kearcher did a few in the past and they came out very nice.
Did you by chance see them?

Firearms imports, consignments


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