Pits are stress risers/concentrators
Light reading here
http://onlinelibrary.wiley.com/doi/10.1002/9780470211106.fmatter/pdfFor us non-deep (pun intended) thinkers, scroll down about 1/2 way for pit-like illustrations.
However, the MWT at the bottom of the pit does not necessarily determine bursting pressure.
As said, Birmingham Proof House requires removal of pits prior to re-proof
http://www.gunproof.com/Proofing/proofing.htmlProof regulations require that shotgun barrels shall be “struck-up” and smooth and that insides shall be clean. Pitting should be removed so far as is practicable, bulges knocked down and dents raised.
