In 2006 the British Rules of Proof were modified to apply increased pressure further down the tube
http://www.gunproof.com/Proof_Memoranda/RULESOFP.PDFPart IV, The Proof Load, Number 27, Part A
Calls for a load of 30% over mean service pressure at a point 17mm or 25mm (about 1),
and at a point 162mm (6.38) from the breech face a load of 30% over mean service pressure (at that point as determined by the Proof House).
The target pressure at 6" and powder used is a Proof House secret
1929 Hunter Arms Proof Load Pressure Curve Chart
http://library.centerofthewest.org/cdm/singleitem/collection/WRAC/id/8149/rec/10712g 2 3/4 was proved with 6.5 Drams FFFg with 1.687 oz. shot = 14,200 psi at 1; pressure at 6 was about 9,500 psi.
Pressures were measured by crushers (LUP - Lead Units Pressure) and modern piezoelectric transducer measurements
would by 10 14% higherA barrel that is dented 6" from the breech received significantly more force than one dented 6" from the muzzle; likewise a bulge 6" from the breech would be a major concern, 6" from the muzzle a minor concern. Make sense?
What the metal deformity looks like microscopically with either a dent or bulge requires a photomicrograph, with destruction of the barrel.
I would suggest "minor" or "major" is primarily a function of the location of the defect, and the wall thickness of the tube at the defect. And that can be measured without sacrificing the barrels.
