When I was young and dumb and full of it, I hired out to shoot off fire works displays, big stuff some as large as 15". They pack one hell of a charge and most of it is black powder.
To make a long story shorter, the supplier sold us on some Chinese stuff, a lot cheaper, and we found out why.Some of the smaller color shells and lower power stuff, we made a mortor tube box out of 3/4" plywood with the 4" tubes seperated by sand poured into the box.
All the fuses on the shells have a protective sleeve over them to prevent unintended ignition, so ever time you shoot one you have to pull the sleeve which I had done and lay the lit punk stick to it and turn your back to the box, and wait for the shell to ignite and get airbourne before you go to the next shell .
I heard the shell lift off and was just about to turn around when the other 5 went off in the mortor tubes.
Splinters, sand, unburned star material all in a large cloud,felt like some elephant had set on my chest, blew a big goober of snot out of my nose
( thought it was my brains), picked splinters out of my scalp, back, legs for a week, took just about that long to hear any thing above a shout.
Don't even mention Chinese and any thing that explodes in the same sentence