Looking at the shell it looks like a classic high pressure problem. Primer and base both show major deformation. I doubt that you changed powders and cause it. Light loads do have a lot of extra space in them, but you would expect to see a major overload. But even if you can not accept that it was a load problem, it was. Barrel failure would not have caused the primer cratering, or the rim to deform like that. Only increase pressure causes metal to deform not the sudden release of pressure by a barrel failure. Rule out powder substitution by cutting open shells to check for uniform powder weight and type. UNles the loads were shot in extreme cold I suspect shell failure not gun failure. Extreme cold can cause weird powder burning sometimes.
You hit upon the most likely reason for the failure. Reloader error. Not you, but the machine. Not here to trash PWs. Great machine, but not absolutely fool proof. I know that all reason and logic tells you the PW is fool proof, and can not drop a double powder or a load and half powder in any shell and get it to close. Been there and done that as they say. It does happen and can be done in light loads easier than regular loads.
If you want a like new PW 900, in 20 ga., go to the East bound Route 50 bridge, over the Chesapeake Bay, dive off the south side, into the shipping channel and you will find my old one. Once every thousand shells I would get what was a very heavy overload. Not a true double powder drop maybe, but a load that would loosen those back fillings. I sent the machine back to PW two times to have it gone over and checked. They claimed to have rebuilt and checked every part on a machine that had not loaded 10,000 shells. Sent it to a PW seller who knows more about PW's than the factory. Lent it to a buddy for a month, until it did it to him. No one ever figure out what, or why, or even how it did it. But three different people, had that machine do it to them. PW assured me the problem was operator error until all three of us explained that the machine did the same thing for us. They refused to admit that a problem existed and could not repair it to prevent it from occuring again. They did take several hundred dollars of my money for "repairs" and "shipping cost" were several hundred more.
Now I did not throw the machine off the bridge because I was mad. I did it because I never wanted anyone else to use that machine again and have the same problem. I shot what must have been proof or above proof loads multiple times without a major gun disaster. But I was luckier than a IL politician with a tape machine turned off. I tossed a few hundred dollars away and never been happier. Best news for you is that it sounds like your son was not hurt. Sorry for the gun loss.
