I appreciate everyone's (well not quite everyone) thoughts. This is a close up of the area in question. It is unfortunate that the missing chunk of barrel could not be found
So just what do you think the missing piece could tell us that the photo of the very thin section doesn't already show?
It is highly doubtful that a chamber lengthening reamer would have enough lateral force exerted on it during a minor lengthening process, to create the thin area.
Since there is no other pitting or corrosion apparent in the remains of the chamber, it is highly unlikely rust or corrosion was involved. More likely... The chamber was probably thin from the beginning, and that thin area just happened to include a small inclusion of rolled-in scale. It happens, and should be a stark reminder that our fluid steel isn't necessarily homogeneous and free of internal defects, just because we can't see them. Even the best steel mills in the world, with excellent quality control, sometimes make scrap. Sometimes that scrap makes it out the door. Metallurgical testing of the remaining 99.9% of this blown barrel might not reveal another hidden defect.
It seems apparent that there was also some prior leakage of powder gasses due to this combination of defects, and that was causing a separation of the rib braze joint. When enough surface area of the braze joint was compromised, it finally let loose. It seems unlikely this barrel was ever proofed with a double the normal pressure load, or it probably would never have made it out the factory door. But it is likely the braze joint added enough strength to this thin area for all these years to contain normal chamber pressures. The tensile strength of a good braze joint can approach the strength of a welded joint. I think that short section of rib was essentially a patch over a thin and defective area that finally failed.
