So, as I clearly did not understand what the issue is, I sent a note to Aaron Little. He replied that the OD where the sleeve would be was not enough to sleeve a 12 bore. He noted that a 16 bore would work, but would have to be thicker and therefore heavier, especially to keep the old forend. Imagine a 20 would be even heavier. So, looking elsewhere to make a new set of barrels seems the better choice. Plan C would be to just sell the action, a very decent high end Merkel hand detachable sidelock with Arabesque engraving. Hopefully it won't come to that...

Regards, Tim