"Run away! Run away!!"
That's pretty much what I've told my 13 year old Son if he's anywhere (other than a well known public event with professionals) someone's firing a cannon. How often do we see a YouTube where we can say, "Prefectamundo. They did everything right."
Too many variables in bbl material, build, powder type, charge weight, wad/projectile, loading procedure, etc. I imagine that at least 30% of casual cannon users regularly do things that most of us here would consider very unsafe practice.
I don't play the lottery; one reason is because I believe it to be a tax on the poor. Playing the 'reverse-lottery' with a cannon is a tax on the ignorant where a few pay (or more frequently,
cause others to pay) the ultimate price.