I played with them quite a bit though and they are relatively hard to set off from my experience. It takes more than a few attempts with a hard heel on a cowboy boot to do it, I would never have guessed they could set each other off with the exception of a few maybe back-to-backs in the same bundle, but never a chain reaction... so I stand corrected.
I was pretty dumb when I was a kid, but not quite enough to try putting a bunch of them in a sack and whack it with a rock. The very small amount of charge that is in them is surprisingly strong and Dad wouldn't have been pleased if enough came up missing and he noticed.
Like you, I never had a primer go off unless I intended it to go off*. What I posted is what others here told me.
Primers are considered hazmat (naturally) and the hazmat rules of DOT forbids their shipment on passenger aircraft.
* And I never ever put them in an air gun and fired at a rock.