Late 70's I was 17 and ran a fireworks stand on the Missouri / Iowa border. We set up shop at a small town grocery store. The store owners had a son who was in his mid twenties, big ole boy - drove a new trans-am. I pictured him as the town bully.
He was always in our stand, taking single firework items and setting them off out in the gravel parking lot - after store hours (after his folks closed the store). Now I didn't own the fireworks, we worked on a percentage of the gross amount sold. I didn't like his pilfering, but he was "sort of" our "landlord", and the total dollar value of pilfered items was minor - so we lived with it. Besides, we often demonstrated what the fireworks did, to boost sales. So in this way we convinced ourselves we could justify his pilfering as "business development" and be fairly certain that no broken bones would happen
One evening he pulls up with a contraption that he fabricated. It was a 4" schedule 40 pipe, about 5" in length. To the bottom he had welded 1/4" plate that was about 8" square. Through the side of the pipe, maybe a 1/2" from the bottom, he drilled an 1/8" fuse hole.
He had this big grin as he poured in about 2" of black powder, wadded up and packed in a 1" thick newspaper wadding, then filled the rest of the pipe with gravel from the parking lot. He then came into the stand, wandered around until him found a suitable fuse, stole the fuse and then went to the parking lot and prepared to set it off.
After screaming and yelling at him to move it to the far side of the parking, he lit it and ran like heck. Oh my oh my. Biggest boom I had ever heard. What seemed like at least one minute, gravel started to rain from the sky, and kept coming for a good thirty seconds. Sounded like a hail storm just starting.
Afterwards, he recovered his mortar, and the 1/4" plate had some serious deformity. He later replaced the 1/4" with 1/2". He shot it off a few more times till the local police seemed to always be in the area when he was around. Hey, I had rat on him. I didn't want to die
As near as I can remember, the 1/2" plate held pretty well. Although I would probably use schedule 80 and even thicker plate.