One additional point re sleeving. One technique employed to ensure that the "monobloc" will retain the original barrel alignment is to use a guided reamer to enlarge the chamber end before cutting out the barrel. In this method there is the risk that the reamer will eat out the barrel before the point where the tubes offer an integral round base to receive the new barrel.
And so what? So the new tubes will need to be filed flat on the internal side where they meet each other but this flattened bit will not have the benefit of mutual reinforcment from brazing. It sounds like an arcane detail, but it is at the most stressed bit of the barrel. I have seen a Westley Richards and a Jeffries sleeved this way and the barrel thickness at the flat internal abutment was 60 thou.
I will pick monobloc every time over these kinds of worries.
