Originally Posted by John Ross

Well, yes, but thousands of tests by the powder companies pale in comparison to the hundreds of millions of "tests" conducted by shooters. Hence my statement:

"In my opinion, there have been far too many reports of weird things happening when extra-light charges are used (most of them by cowboy action shooters) to dismiss them all as reloading error. Again, there’s a simple way to avoid any possibility of detonation ever happening to you: Stick with loads that fill most or all of the available case capacity. Want a lighter load? Use a slower powder."

Although I didn't say it, loading this way also makes double charges impossible. And yes, I think a double charge is the LIKELIEST explanation for this event, but I am not willing to say it is the ONLY explanation.