Sporting Life March 30, 1895
http://library.la84.org/SportsLibrary/SportingLife/1895/VOL_25_NO_01/SL2501014.pdf
Speaking of these unaccountable discharges of guns, we are reminded of a case which happened but a few days ago at Willard Park, Paterson, N. J., where the Great American Handicap will be held in April. It was in a match between Captain "Jack" Brewer and Chris. Reinhardt. When the match was about two-thirds finished Brewer killed a bird with the first barrel and without removing the shell from the other barrel he walked from the score, and just as he was about to set down the gun it was discharged, putting the load of shot into the board at his feet. It made slivers fly, as Jack usually shoots a smart load. What the result might have been had the gun been pointed in some other direction we do not care to think about.
In the first place Brewer had no business, as a pigeon shooter of years' experience, to have turned from the score with a shell in his gun. In the second place, the referee should have been more watchful and not allowed anyone to turn or leave the score until both barrels were empty, not even an empty shell should remain.

More on Brewer, who IMHO was a bit of a sociopath
http://www.trapshooters.com/threads/the-best-shot-on-live-birds-the-world-has-ever-known.230843/

I don't know why it's called "an unaccountable discharge ". In all my experiences of unintended gun shots the trigger was pulled. Perfectly understandable. A trigger finger temporary disconnected from a brain.

While not a double barrel, my late friend Cooper had an "unaccountable discharge" on a dove field, years ago. Maybe someone here can explain it, neither he ,or his friends could give a likely explanation. He was sitting on a dove stool with his mod 37 laying on his lap, when a shell in the magazine spontaneously discharged. Thankfully, he was not injured, although very "shaken up". There were about 10 of us on the field, and none of us could explain the discharge in a manned that fit what we were looking at. It could have happened with a double, but if it had, we would concluded, like nialmac, that a finger was involved, when it wasn't. Does anyone have an answer?
Mike

Every unintended discharge I've ever had has involved a finger on the trigger when it shouldn't have been. Of course, a stick or vine can pull a trigger as well, but I've never seen a safety failure.

Upon reflection, I did used to have an old model 37 that sometimes fired the 3rd shell unexpectedly when a bunch of ducks stormed my decoys. I still don't know whether those were 'slam-fires' or unintended trigger pulls...Geo

George,
The shell that fired was in the magazine tube. The case, wads, shot, and all stayed in the tube; we watched him take the end cap off and pour it all out. Anything pulling the trigger would have fired the one in the chamber, not in the magazine. If a chambered shell had fired, it would have been understandable, this was not.
Mike

A shell going bang in the magazine is a new one on me. What on Earth could have caused that?...Geo

I'm gonna go with it was something unaccountable.

Drop a cigarette butt in the ejection port?

The forensic evidence would have been interesting to look at.

Like what did the primer look like, the carrier and action functioning properly, etc.

Maybe the guy loaded a Paradox style slug with the pointy tip into the magazine behind the round that went off?...Geo

The incident was on a dove field, so no paradox ammo, he didn't smoke, so no cigarettes in the tube. He was sitting in the sun but it couldn't have gotten hot enough to "cook off" a normal primer. I can't say for sure it was a normal primer, because something happened that we couldn't explain. While it had happened many years before, he was still trying to figure it out on his hospice bed.
Mike

I have had a double discharge upon closing. It only missed my birddog Ginger by a few feet, and she was just a few feet away.

With further experimentation I got it to do it again.

My conclusion. I must always be very aware of where the barrel is pointed when I am closing a gun.

And I have had a few doubles that the discharge of the first barrel caused the second barrel to fire, including a brand new Huglu single trigger 28 gauge that I won at Quail Unlimited dinner.

I have heard of a pump failure to fully eject a spent shell which was then jammed with its brass rim between bolt and primer on a chambered round and firing when the bolt was pumped to close, but not a spontaneous firing of a round in a magazine.

Here, according to Mike, we had a model 37 pump resting on the owners lap when a shell in the magazine tube discharged. Mike gives us a bit of a clue when he says this happened a long long time ago. All of the forensic evidence is long gone, so about all we have is conjecture and old memories as clues.

I had thought about a "cook off" scenario, but realized that usually happens after extended full-auto fire when the barrel and chamber get extremely hot. Even in the hot Alabama sun, it isn't likely that you'd get a pump shotgun that hot... particularly in the magazine tube. Too bad we can't see the shell casing to examine if there was any damage to the primer. It's doubtful if the powder spontaneously detonated, but the old primer could be a different story.

It wasn't until after 1935-1938 that true non-corrosive non-mercuric (NCNM) lead styphnate priming compound mixtures became pretty much the standard because it was much more stable and much less corrosive than the potassium chlorate and mercury fulminate compounds that had been used previously in primers or percussion caps. Mercury fulminate, in particular, had a nasty characteristic of being so unstable, that it would spontaneously explode for no apparent reason. Since primer manufacturers were hard at work experimenting with different compounds that addressed the problems of instability and corrosion, it is entirely possible that the shell which detonated in the magazine of Mike's friend's M 37 had an old primer that was composed of a mixture that was unstable enough to just explode in a magazine tube on a sunny day. Mike's description of how the wad and shot remained inside the magazine tube sounded like what happened when I got the brilliant idea to put some of my Dad's shotshells into holes poked in a cardboard box, and shot at the primers with my pellet rifle. The shells would go flying, but the heavier shot charge simply dumped into the box. Stuff like this is why so many teenage boys never make it to adulthood.

Anyone who has spent enough time in an organic chemistry lab can tell you just how sensitive and unstable some compounds can become under the right circumstances. In college organic chemistry, my classmates and I played around with nitrating just about anything we thought might explode. Once, I made some nitrocellulose, or gun cotton, and took it outside to dry on a rock in the sun. When I came back later to get it, all that was left was a scorched mark where it had rapidly burned. We also played around making ammonium triiodide and nitrogen triiodide, which are highly sensitive contact explosives that would probably get you in trouble with Dept. of Homeland Security today. The ammonium triiodide was fairly stable as long as you kept it wetted with acetone. A friend had some in a small vial in his apartment, and the stopper apparently was loose allowing it to dry out. Someone slammed a door in the neighboring apartment, and it blew up causing minor damage. I had a couple minor explosions in the lab just distilling organic compounds in a reflux condensor, and allowing them to boil dry in the flask. We started making a batch of nitroglycerin, a very simple process, but thankfully lost our nerve. Coincidentally I think, some of these misguided experiments came when we would have cocktail hour with ginger ale and anhydrous 200 proof ethanol when the professor left for lunch.

Keith,
The incident was a long time ago, but not that long, it was about in the mid 90s.This was about the same time some manufactures were having trouble with consistency in primer production. The primers had to be non-corrosive non-mercuric, but there is no way to say they were not unstable, for some reason. I can say it was factory ammo, since he wasn't a handloader( unusual in our crowd). I have seen primers seated upside down in factory ammo, and sideways in my handloads, and they didn't detonate, but he would have noticed that when he examined the shell case afterward. Something made it happen. I guess if you live long enough, you see a lot of things.
Mike