This Spud Gun I designed to be able to use hydrogen as a fuel source. The combustion chamber is made from pressure grade PVC, which is reinforced with 3 layers of fiberglass to reduce the risk of a catastrophic failure.
I have witnessed a few Spud Guns explode due to inappropriate materials and excessive size of the combustion chamber. The pressure inside a Spud Gun can be between 100-150PSI depending on the fuel to air ratio.
So as the diameter of the combustion chamber increases so does the area of the end cap... So the question you must ask yourself is the end cap going to be able to take 150+ PSI (Pounds Per Square Inch).
My Spud Gun also has an electric 12 volt PC fan inside the combustion chamber to assist in removing the exhaust gases. I notice some people use their mouths to blow out the exhaust , this is a highly dangerous practice! The exhaust may contain unspent fuel which could ignite if a spark is produced accidentally, for example from the ignition button being pressed or static electricity. I have actually been able to fire multiple shots without adding new fuel, but I have only been able to do this a few times. The ignition system is basically a circuit that I removed from a plasma globe, which conveniently already runs on 12 volts. I added a larger heat sink to the main power transistor to absorb the extra heat that will be created. The extra heat is created because the arc on the spark plug is actually creating a short circuit in the system hence the extra heat. I used a small spark plug in the end of the combustion chamber, which is screwed into the PVC end cap then fiberglassed in place.
I have two different sized barrels which are easily screwed off, one shoots smaller shots which I usually use for shooting potatoes. The other bigger barrel can shoot larger potatoes which will not go as far , but it is mainly used for tennis balls. The Spud Gun can be excessively loud which gave me the idea of making a silencer like what are used in the movies on handguns. I didn't really do too much research into the design, it was just something I thought up while on the long drive home from work. It consists of 4 baffles glued in to the PVC joiners which I also used 4 of, the baffles are made of MDF. A couple of the baffles did sustain damage from a rouge potato, but still function fine.
If I were to do it again I would use a harder material than MDF. The whole silencer cost about $50 AUD to make, the joiners were quite expensive. But all in all the silencer did make a quite noticeable reduction in noise from the Spud Gun.
I did get to use it on hydrogen a few times, which I generated from my hydrogen production plant. The potatoes that I used disintegrated as they left the barrel, and as it fired the bang was even more deafening than usual. I ended up using a tennis ball in the end, that went significantly further than when I was using deodorant or petrol as a fuel.
I chose to use hydrogen because of how explosive it is and because of the flexibility of the air to fuel ratio. Hydrogen can still ignite in the combustion chamber even if there is an excessive amount of hydrogen present which would normally be two rich if any other fuel was used such as propane or petrol. Please use common sense if you are to make one of these and never point it at anyone even if it's not loaded!
Video at the bottom of the page.
In the video the spud lands approximately 130 meters away.