Thanks Mike. I think I understand what you're describing. I was aware of a choke tube fixture for doubles that Colonial made. Jack First listed in an old catalog, but they do not have one.

One thought, probably flawed, was to use a reamer large enough to ream the monobloc by hand. Not sure if there are reamers for that use that would large enough. I'm assuming you'd cut enough to erase the rim cuts.

If I can figure out a strategy, and find new tubes, I would be permitted to do this at school.