I think you may be confusing "sleeving" with "lining". When I think of sleeving I think of barrels cut just past the chambers, with a portion reduced in diameter and slipped into a new set. New barrels on old chambers. Lining, as with Briley full-length tubes, does reduce the gauge and balance but enables you to shoot a 20 gauge in an old 12 that may be unsafe for 12 gauge shells. Teague liners are another matter entirely.