When I see sucha used guns I understand proof law is a very good thing, my friend.
There is special term in Russian and in German: "different-wall-thickness" in one word. Usualy as result of many times re-bluing and honing using hand tools or tools for drilling machine.
Every time you shoot barrel breaths and this weak spot on barrels breaths also. And in one moment steel in this place becomes tired and you get what you got.

Geno.