To sleeve the barrels, the (intact) breech is bored out to act as a monobloc, into which the tube extensions are soldered and the seams TIG welded
If the chamber has burst, the barrels will need to be disassembled then re-fabricated from scratch. The good tube, ribs, top rib extension & lump could likely be used.

This is from Ladbrook & Langton Gunsmiths site

[Linked Image from photos.smugmug.com]