All good fun, but I can't see it being legal. It's essentially a muzzle loading firearm. Projectile, burning propellant, considerably more than 12ft-lbs of energy even if you disregard the fact it's powered by an explosion. I wonder if it's ever made court?
I made a basic musket when I was a kid, 12-13 years old at a guess, from hydraulic pipe threaded to take a bolt as a breech plug. I'd guess about 3ft long. I drilled a small angled hole in the tube at the plug end and used to use fishing line to pull 100 paper caps taped to a party popper charge down the bore and through the hole. The string fitted through, the charge didn't. On top of that went a Barnett steel catapult bearing patched with a piece of rag. I'd hold the pipe tight, sight down it like a shotgun and pull the string. I never found out what would stop it, it smashed straight through an offcut of 9x2 so I tried a railway sleeper. All in my back garden, all went through the fence never to be seen again. Thankfully I guess they must have stopped somewhere safe because I would have heard about it otherwise, and the breech plug didn't fail because it was pointing pretty much straight at my eye!
Folk say "kids these days!" but kids in those days weren't exactly innocent either!