Henry,

Yes, I'm wondering how to record the premature detonations as a cause of failure. Honestly, I'm somewhat skeptical of these actually being the root cause. In a number of cases they may merely have been the "straw that broke the camel's back." The reason I hypothesize that is the bursting charge was typically 1/4th or less of the bursting charge. I can come think of some reasons for this to be sufficient and to have much more impact than the powder weight alone suggest (particularly farther up the tube where the pressure curve is supposed to be far less.) However, I would expect that to be within the design margin too. So if the metallurgy/casting of the piece is already marginal and it is weakening with each firing cycle, then a premature detonation would be sufficient in some cases to cause failure much sooner.