I do not know about your tanks but I am a firefighter and our tanks-the old steel ones-run at 2100 PSI or so. They are tested in a burst chamber at 5000 PSI as to rate of expansion. I agree that the pressure would be way too high for the wall thickness to be effective. I have a friend who made a cannon/mortar out of a piece of 2 inch 5000 PSI steam pipe. It has a wall thickness of 3 inches. He taper cut the wall at about 40 degrees and used about 15 LBS of welding rod to weld the cylinder to a 5/8 inch thick 16X16 steel plate. This one is safe to detonate from a distance----Yeah a full pound of black powder to a loading with a large paper bag for the wadding and the thing gets driven about 8 inches into the ground when it goes off. You do not want to stand closer than 50 feet to it due to the concussion/flash when pointing up.
These are older steel tanks in good condition made of chrome molybdenum steel, not aluminum. I was a paramedic for years before becoming a PA and am aware of the Scott packs used by fire departments. We used them too. Like scuba, the older ones were steel, aluminum has pretty much replace all these tanks in recent years.
My tanks are 72 cu ft tanks with a working pressure of 2250 psi. When they are hydro-tested, they are pumped up to 5/3s the working pressure and can't expand over a given amount at 3750 psi. This is not the burst level. I'm not sure of the burst strength but would be at least >3750 psi.
The mortar on the Pawn Shop episode was the same type of steel, only a bigger tank. The dive shop we use to patronize had a bank of these "M" size cylinders used to cut down on time to fill tanks.
My imagine is to cut 1/3 off the base, not using the valve end but the round end with a touch hole drilled resulting in more of a mortar look than a cannon look.
I like the idea of a sabot of lighter material encasing a denser mass. The diameter of my tank is more small cantaloupe size than bowling ball. Maybe a frozen tomato with a sabot around it might work. I saw an episode of Myth Busters where they used frozen chickens with an air cannon. Trouble with air cannons is the release valve is huge, plus there is little or no bang.
My neighbor has a much lighter apparatus than my proposed tank outfit powered by propane and shoots tennis balls. In my wayward youth, I made tennis ball cannons out of beer cans when they were not made from flimsy aluminum. Have to use bean cans now I guess. The way my beer can cannon worked was to cut the top and bottom out of about 5 cans, after drinking the beer. At that time in my life poring out the beer to get the cans was a no-no. Cut maybe 1/3 of the top out of one and put a small hole in the bottom using a church key. This was the breech or expansion chamber. Wrap the cans together with duct tape covering the back three cans with 4-5 layers of duct tape. Squirt about a cc of lighter fluid through the small hole in the breech portion of the thing. Roll it around good to get a fuel/air mixture, touch a cigarette lighter to the touch hole. Getting the fuel/air mixture right was the key to how far the tennis ball went.
The propellant on my proposed scuba tank mortar and how much is the question. I'm a bit concerned about using very much black powder though starting real low might be an option. Definitely no modern powders. Propane is a thought. The projectile would need to be something other than metal. Frozen vegetables are top on my list.
The Pawn Stars cannon had a re-enforcing ring around the muzzle or cut end. This must be needed to keep the muzzle from pealing back form a ragged cut.
The carriage for the thing would need to be robust beyond what would reasonably be needed to be safe. The guy in one of the above links using a folding chair was comical.
Thanks for the advice. I doubt I come up with a marketable design but might be fun trying.
