[pvg]

I love that the tremendous ambition of this is what kept it non-functional and relatively safe - the home-made gunpowder is probably not fine enough to blow up the very sturdy CO2 cartridge, the hole at the end of the cartridge is probably too big, etc.

You can make an equivalent functional microscopic 'pipebomb' with just matchstick head scrapings and the plastic body of a pen. This usually does produce a small satisfying bang and the risks of screwing it up are mostly limited to 'very nasty burn'.

[hilbert42]

I don't want to sound like a spoilsport but any extension of that to say water pipes with capped ends is in a different league altogether. Leaving the unlawful aspects of it aside, matchstick heads usually have powered glass in them for friction to generate heat. If any material gets on the screw threads when screwing the caps on then I don't have to mention the outcome. (It's happened many times before.)

[MertsA]

As a cautionary tale for would-be amateur explosives aficionados out there, you get a similar problem storing a lot of the more accessible energetic compounds as well. It can crystallize under a screw on lid and blow your hand off if it doesn't kill you outright. It can degrade and become more and more sensitive. There's plenty of news stories about teenagers blowing their hand off because they thought Acetone Peroxide was all the rage, but it's easy to overlook that while it appears simple on the surface, there can be plenty of hidden hazards that are unintuitive. You don't know what you don't know and the learning process can leave you pulling your socks off to count past 7.

Stiff cardboard tubes, packed clay, and low explosives like plain black powder, flash powder, etc provides all the confinement you need for a bang and a flash. Thermite is also fun, and so long as you're not igniting it on top of a car battery still filled with acid, it's quite a bit safer.

You can also go the slow pyrotechnic route with making smoke bombs. It's not as simple as it may appear, but from a hazard point of view nothing is going to explode. So long as you take some basic fire safety precautions like doing it outside away from structures and flammable materials on a hot plate it's safe and entertaining. Couple of safety tips, if you're grinding up your oxidizer in a coffee grinder (definitely recommend, mortar and pestle suuuuuucks) make sure you never use it for anything other than oxidizers in the future. If you use it to grind up a fuel later on there's a very good chance you won't get every last trace of oxidizer out so you could wind up with a bit of powdered fuel and oxidizer mixing together in the tiny joint between the blade and the bowl and completing fire triangle bingo. Another foundational precaution is to have a good understanding of what your ingredients are because getting it wrong can be quite hazardous, not just disappointing. For example a friend of mine in high school wanted to make some smoke bombs and was asking me for directions. I told him to find some stump remover from the hardware store and make sure it was plain potassium nitrate. He couldn't find it on the bottle and assumed it probably was and figured it would either work or it wouldn't. That bottle was filled with sodium metabisulfite instead of potassium nitrate and when he tried to heat it with sugar to melt it together it started decomposing into sulfur dioxide gas which combines with moisture in mucous membranes to form sulfuric acid. He had chemical burns in his throat and was coughing for a bit afterwards but luckily no lasting damage.

Another fun misadventure with the same friend. We came up with the brilliant plan of stuffing an entire gross (144) of bottle rockets into a huge pickle jar that was the perfect height to hold the body up over the mouth of the jar. We sprayed the fuses all over with starter fluid (ether, not lighter fluid), lit the bundle, and ran back. Moments after they started launching the heat from the exhaust (and the ether might have been a contributing factor) shattered the pickle jar and now all of the remaining rockets are splayed out in every direction horizontally. We ran and dove with rockets going everywhere and these were the ones with a healthy report at the end too so it's basically just launching fire crackers at us.

I'm not going to try to tell anyone not to play with fire, but please show some modesty and act responsibly. Everyone makes mistakes and overlooks things. Plan for this, expect it, and position yourself so the fallout for things going sideways doesn't involve the fire department or the coroner especially when your latest escapades involve showing your friends. They're idiots, they will do dangerous things out of ignorance. Don't put someone else in a position where they need to know anything other than stay behind that tree and don't come out if it doesn't go off. They aren't going to stop drop and roll, they're going to panic and make things worse like running with a flaming gas can spreading fire all over the place. Aggressively plan for failure because things will go wrong and you need to be able to deal with that when it comes up.