Miller,
I took your level of concern as you stated. I was interested in what information you had on the issue.

I did think you were referring to vintage fluid steel barrels. I thought you might have a specific reference book citing data that quantified that the problem.

Yes, I have cut into flaws in modern materials. But, the statistical probability of encountering one in modern American made steels intended for the aircraft/aerospace industry (mostly what I've worked with) is extremely low today. Of course, for higher criticality parts, vacuum arc remelt steels are usually specified today, and this reduces the probability of inclusions and flaws further.

For the record, I spent a total 15 yrs actually cutting metal as a production machinist, experimental machinist, toolmaker, etc., including having owned a small machine shop, plus a 10 yr stint in quality assurance at varying levels from actual inspection of machined parts to quality manager, quality engineering and failure analysis. My current job has me looking into results of lab testing and failure analysis on occasion when it pertains to regulatory issues on airplanes under my responsibility in my airworthiness function.