This guy should not be allowed near live or presumed inactive HE devices.

It sounds like a 40mm cannon payload, as opposed to the entire cartridge. A teacher a few months back blew his hand off doing something with what was identified as a "40mm shell" and turned out to be a 40mm grenade launcher cartridge. The primer was struck when he pounded it on the desk, and the gunpowder inside exploded, splitting the case and injuring his hand.

A 40mm cannon shell is waaaaay more potent than the 40mm grenade. It sounds like he removed it from the case, and discarded the powder and primer along with the case, keeping only the explosive warhead! Of course, we all know how painstakingly accurate all journalists are...

Thing is, those shells usually have a centrifugal safety that prevents the shell from arming until it has spun so many times at a certain speed (so you don't shoot a piece of whatever flying by the end of your cannon and blow yourself up). Were the kids using it as a top or what? They couldn't spin it fast enough. They may have been trying to do something odd to it, like set it on fire.

Stupid man doesn't deserve to be alive while those kids are dead. I'd acquit if I was on the jury of the childless father who killed that man.

I'm as far from an expert on military explosives as they come, but how recently were those safety measures implemented? Might this shell have been before that era?

Oh, somewhere after the Civil War. The very first impact fuses were spin-time-distance safed. That's not the official term, obviously. I'm fairly certain they were used in the spanish-american war. Before that, an artillery shell cosisted of an iron ball full of black powder with a small hole drilled into the center where a literal fuse was run into the center. The burning cannon powder ignited the fuse and the cannon ball hopefully exploded once inside the target.

Hmm, you know just a bit about this stuff...

I just kind of assumed it was common knowledge.

There's probably a lot of things I know that most people don't that I would assume are pieces of common knowledge.