I have fourty-seven years of reloading experience. I've probably reloaded over a million rounds of ammo. I have one thing to say about this situation: There but for the grace of God go I!
Reloading is a hobby that DEMANDS 100% attention at all times, but it not only requires attention when the reloading is done, but when test loads are fired. From this distance it is very hard to tell what exactly happened. It was probably a double or triple charge. I do not, however, discount the idea of a detonation or of a stuck bullet followed by another round (though with a stuck bullet it is usually only the barrel that bursts). People are human, however, and you are fooling yourself if you think you can reload forever without a momentary lapse of attention. You need to give yourself every chance. By this I mean set up your reloading procedures to minimize the chances of screwing up, and maximize your chances of catching yourself if you do. Like others, I prefer to use a powder where double charges are likely to be obvious. With a single stage press I have my powder measure and press set up so that I charge my case, immediately look into it with a good light, then immediately seat a bullet. If you line up a bunch of cases in a reloading block and try to charge them all, eventually one will get no powder and the next case will get a double charge. Don't think you will always catch this by looking into the loading block full of cases with a flashlight. We have all seen the paragraph written where they ask you how many "f"s you see in the paragraph. Usually people miss the count. This is because your mind sees what it wants to, or THINKS you should be seeing. The same thing can happen when you look into a reloading block full of charged cases. The way to count the number of "f"s in the paragraph is to start at the end of the paragraph and count the "f"s without reading the words. This helps your mind see what you need to. This is also why you look at each case one at a time, then seat the bullet. Your mind can concentrate on ONE thing, and you are less likely to see all cases charged to the same level.
If you have the SLIGHTEST idea something was done wrong, pull the bullets, dump the powder, and redo it! Yes, it is work. Yes, it is frustrating. It will save you much embarassment and a destroyed firearm, however. Don't EVER think it can't happen to YOU, however!
THERE BUT FOR THE GRACE OF GOD GO I!
Whelenshooter