Good discussion here
http://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=357105#Post357105 And if I might rant for a moment 
2-3 times/year between this and other forums, someone reports a blown up gun/barrel, might post a few poor pictures, and the usual experts (including myself) pontificate as to etiology. The OP and the remains then disappear and we have no follow up and are no smarter.
To properly diagnose a blow up requires a formal failure analysis and metallurgical study, by an experienced examiner, and costs about $1500 done right. But unless a personal injury lawyer gets involved, it won't happen, and if the case goes to litigation, we'll likely never know the results.
I believe however that MOST
vintage gun barrel ruptures can be diagnosed simply by looking for a bulge and
measuring the wall thickness. How hard is that?
