Did that one hold together? I would've fired it remotely with a video feed, after simply packing some mud in the muzzle of a standard chambered rifle that was destined for the scrap heap anyway.
Stuff like that really impresses kids. I remember back in my first gun safety class the instructor laid a .30-06 cartridge on a hot plate covered by a big cardboard box. When it cooked off the shrapnel failed to penetrate the walls of the box. Good thing, as we were all crowded around it to watch. Good times.
