Some of the comments concerning sleeving leave me to believe my description was insufficient in explaining what occurred with the gun. This is not a bulge or rupture in the bore. It is a total blowout of one chamber, leaving nothing to use to sleeve a new tube onto. Heretofore, I have always believed that the only option was for a gunmaker to disassemble the barrel set, solder another sound tube in place, and relay the ribs, then blacken it all. That, or find the needle in the haystack set of abandoned barrels that could be refitted to this action. My purpose in the question was to be sure there were no other options that were available.
I do understand, as Stephen rightly pointed out, the cost of having a new set of barrels made would be extremely prohibitive.
I need to get that Boswell off my mind totally, I guess. It just irks me, even some twenty years later, that the owner was such a know-it-all that he loaded the shells with heavy charges of Unique and compression-formed hulls, because "he has plenty of that already on hand". He maintains to this day that a base wad separated from the hull's base and lodged in the bore causing the blow-out on the next shot. No amount of explanation can dissuade him from that belief, even though I know better.
Mike, would you kindly point me in the direction of some good pictures of the Lang which you have for sale?