There's actually a Short where they try to push up a nail up a cartridge round and see what damage they would do. And at point blank the cartridge managed to blow a hole through a thin metal. So I guess you're prolly wrong, if it can chew through thin metal, it sure would pierce through boots.
Yes I see your point. Having a boot sized pit would give too much room to disperse the blast. How I would do it is with chewing gum mixed with ground matches and striker powder, as the ignition to the base of the cartridge. That will make it very, pressure and friction sensitive. Put it in a short pipe with a stone at the base crimped off. If no pipe use a rolled up can or GI roof sheets etc. Stick it snugly into the ground and it's ready.
