I’ve seen a round go off when it was dropped onto gravel…scared the crap out of us, but not a big deal; just a loud “pop!” and the case splitting, barely pushed the bullet out (it was a .45).
One in a million, and that’s probably on the high side of the odds.
A couple years ago on my old agencies line a guy dropped a factory box of Federal 9BP 115 JHP ammo and a round detonated. Chewed up part of a corner of a the box.
The cartridge case was in pieces like one would be outside a chamber that detonated
i was told that could happen, in fact it was at the state owned open range..they tell you to load your magazines, then place the box(es) under the bench...
the RSO said that "could happen"
i also saw another video on this, if i can find it, i will post it.
it's a freak thing, that the shell casing just happens to hit the primer on the rounds on the bench, or in the cartridge holder from the boxes.