I read a post on Say Uncle about a collector who blew himself up with a Civil War era cannonball. I started leaving a comment there. It got so long; I turned it into a post. There that gives me something to write about!
I used to work in a history museum. People sometimes brought in "cannonballs" they had found for advice on cleaning or to donate or sell to the museum. Once in a while, they would bring in a shell.
A shell looks like solid shot, can be fired from the same cannon, and once corroded is difficult to recognize. A Civil War shell is hollow and filled with black powder. It has a hole in which a gunner placed a wooden cone that had a fuse inside (pointy-side into the hole). The base had numbers on it and little holes drilled by the numbers. A gunner could prick a hole by a number thus cutting the fuse to set a rough time.
They would load the shell into a cannon with the plug facing out. If you placed the fuse against the powder charge and fired the cannon, you'd end up with a Wile E. Coyote shredded cannon (if you were lucky). The fire from the cannon blast was supposed to "wrap" around the shell and ignite the fuse. Relying on the fire to act like this is not the most reliable ignition method. When it didn't work, the shell acted like a cannonball, did whatever damage it could to whatever/whomever it hit and then settled into the dirt. Many duds have stayed intact over time.
Cannonballs are collectable as is any Civil War artifact. Someone finding one decides to clean the dirt off and sees only a rusty blob of metal. Some use grinders to polish it up. A few get out a propane torch and heat the rust in order to scale it off. Occasionally, their work does what the cannon blast failed to do so many years ago.
Black powder has a long life expectancy. Even if it gets wet and doesn't get washed out, it can remain potent once dry. Granted, a lot has to happen for a shell to go off: it has to be found, it has to be cleaned, it has to have an intact charge and fuse, and it has to be exposed to heat/fire/spark. As the story illustrates, all of this can happen even after 140 years.
When shells came to the museum, we explained to the finder what they had. We photographed them, measured them, wrote down the finders' stories, and then turned them over to the bomb squad. No collector in my experience disagreed with that decision.
Once in a while the bomb squad would let us watch the bang. Since they placed the shell in a bunker, it was not too exciting. Still, it was enough to prove that ordnance is nothing to fool around with.