Might as well mention "Zircon's" metallurgical failure analysis on the Sherman Bell Parker VH fluid steel and GH damascus barrels. His full report and the images are copyrighted, but this is his post from 2007

http://www.familyfriendsfirearms.com/forum/archive/index.php/t-55364.html

The extractor screw hole acted as an initiator and low cycle fatigue did occur with increasing load pressures until cleavage