I would not conclude there to be a safety factor at the .050" thickness, remember it "Bulged" with a heavy service load, not a proof load. I think that the Germans in their recommended minimums (.090" for ordinary steel) were allowing for a safety factor, at least enough to pass proof.
I would bet though if you knew the tensile strengths of the bbl Greener tested & used most any common formula for determing necessary wall to hold the pressure of the load used it would come up well above the .050". I would also highly suspect if you could have put the same pressure as the max of that load in as a static load it would have bulged far worse or even burst. That is another slight built-in safety factor, most formulas are run based on static pressures, but the duration is not long enough for the full effect to take place.
It is also noted that General Hatcher reported in his "Notebook" on turning the wall over the .30-06 chamber of a standard 03 Springfield down to 1/16" thickness & it holding several service rounds without bulging.
These were of course all done as tests, I prefer a little more meat over the chambers of guns I plan on putting to my shoulder. I would not feel comfortable firing those shotguns with chamber walls down in the .06x's "Period", the "Safety Factor" has been mostly Cut Away there.
