rh44 - yep, you got that right. The reasons to set barrels further apart are to allow thicker breech walls or to fit to existing firing pin spacing. Per my concept that monoblocking doesn't require additional barrel spacing width, the practice of sleeving is little more than monoblocking the barrel set. It is practiced on all methods of barrel joining that I'm aware of and without any need to adjust barrel width.
When a barrel set is prepared for sleeving, the tubes are cut off and the breech end of the set reamed to accept a new tubes with the breech end sufficiently thick for chambering. The tubes can be glued in, soldered in, brazed in or welded in.
