Another hypothesis. A few years back, I borrowed a few reloaded ammo for my "new" 16 gauge... I know you don't shoot someone else's reloads, but they came from a very competent reloader and it was use these or watch the ducks fly all over. The first shot literally put me down on my butt, the gun jammed close and the hull spoke of a very high pressure. After that, I did a very crazy thing, that can only be explained, but by no means justified, by the thrill of the hunt. I watched one flock of ducks after another pass over my head, and then thought, well, maybe it was just that oddball shell, double loaded or something, and the others were OK? I fired some more of these shells, and they seemed OK, and then I had another KA-BOOM, not so dramatic that time, but still obviously high. That brought me back to my senses, sort of. At home, I disassembled the remaining shells, and they looked absolutely OK, no double charges or anything. And yet two of the batch produced overboard kind of pressures.
It looks like the powder the shells were reloaded with got into some chemical reaction with the residue from the powder originally fired in the shells, or with some outside contaminant. Actualy, the idea is supported by my Dad, who's been 20 years in industrial chemistry, and he says when there's high pressura and temperature involved in a fine chemical reaction, you never know when the most theoretically innocent contamination will act as catalyst for what process and where it can lead.
