Anyone who says the extractor held the round in place is correct. That's the nature of the world famous controlled-round feed system. From there, I think the bullet flew ahead and plugged the bore, turning the chamber into a pipe bomb. He's lucky the bolt didn't fly through his head.
I don't know if it's on the unsafe list, but .44 magnum ammo will fit in a .45 colt chamber. The rim thickness and diameter are the same (or within 2/1000 of an inch). I think either of my .45/.410 bore Contender barrels would shoot a .44 okay, but I'm not stupid enough to try it. I'll just put the .44 Magnum barrel on and use the right ammo. I also heard that combination will blow up a SAA.