Craig: Tis' the season for good will. We're all friends here and just trying to understand.
You used a term "axial shaft located in the receiver" which I've never heard nor can I find anywhere. Double guns have an "action" and the barrels are not part of the "receiver". Are you saying that Shawn's friend's barrels burst starting at the shaft? Does the gun have a shaft? Do Parkers an axial shaft?
The wall thickness recommendations of Hugh Lomas and
Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives (C.I.P) are here, and you are most free to disagree. Yes, wall thickness of .068" at 6" would be normal
https://docs.google.com/document/d/1...vwLYc-kGA/edit
What are your recommendations based on 50 years as a machinist and having disassembled 30 British boxlocks?
We can all guess at the wall thickness at the burst, but possibly the Shawn could simply measure the proximal and distal walls to settle the issue?