Lets not forget that the purpose of the proof test is to protect the end user from catastrophic failure!
In my opinion the London Proof house is wise to withhold comment on the alleged barrel failures until such time as the root cause of failure is established. Based on the comments by the Birmingham Proof House the problem reported appears to be an isolated issue at the London location. As a matter of interest I am listing below the factors that are likely being investigated both by the London Proof House and the barrel manufacturer.
1. Was the pressure generated by the proof charge correct?
2. Was the barrel material made to specification and was it
free from defects.?
ie: Was the tensile strength, chemistry and microstructure
OK; was there any evidence of subsurface voids or
inclusions in the failed barrels?
3. Were the dimensions of the barrels [ie; bore,chamber,forcing
cone and wall thicknesses made to specifications?]
4 Was there any evidence to show that incorrect brazing
proceedure contributed to failure?
5. Was there any evidence of longtitudinal scratches in the
barrels caused by tool withdrawl marks?[ Ie; a stress raiser]
6. Was the correct test proceedure followed?
7. Was the gun properly prepared for test?

REMARKS..
It is my understanding that the proof house uses [or used]
a pressure barrel utilizing the metal crusher method to audit
proof load pressure.
Did the use of double proof tests play any part in the
failures reported?
The Birmingham Proof house is to be congratulated for their
frank comments on this issue.
I am sure all involved, users and industry will look forward
to a report identifying the cause[s] of failure from the
London Proof House.

Roy Hebbes