When I was 14 back in "Jolie olde" I made a cannon out of 1" mild steel in metal shop, about 7" long,3/8" bore. dumped the contents of a "Brock's Cannon" banger (Fire-cracker) in it and a large split shot paper wadded. Used my gym bag as a catcher to see if the projectile would penetrate or dent it. Upon firing the cannon blew back about 30 feet, the split shot killed my bag, went through my gym shorts and set them smouldering travelled into my math text book about 3" edge-wise in 2 pieces ruining the book. This first firing was in a stone-bridge under-pass to get max echo, our ears rang for a helluva time, didn't even hear the broad #####ing at us for awhile. So I guess it's like BP, for firecrackers just use Pyrodex it makes a good bang in heat shrink tube which is re-usable.