Simple - unless the steel has reached it's yield point (before the elastic limit)there will be no evidence after firing during proof testing of how close it came to fracture. That could be dictated by any residual incipient flaws present. In fact, the British Proof testing methodology could actually induce partial failure. Some would say that it is a form of Semi-Destructive Testing!

In other words - you have no idea of whether the pressure came withing 1% of failing, or 200%!

Even today, the British Proof Houses use proof pressures that are only 30% over the HMSP (Highest Mean Service Pressure) - not much of a margin given the vagaries of temperature, bore size and bullet hardness/diameter/bearing surface etc.

Metallurgy has moved on somewhat during the last 100 years, and today there are a number of Non-Destuctive Testing techniques that Materials Scientists use on say Pressure Vessels that ensure that once the computations of Hoop Stresses are done (basic Engineering!) , the part will not fail as a result of any flaws in the steel. It's a pity that the industry has not seen fit to adopt them!

Cheers!

470R