The technical problems of cannon construction have been exercising aircrew brains ever since Blackadder and Baldrick joined the '20 minuters'. Here's the benefit of my 'ahem' fairly extensive cannon building experience - gained mostly on long stints on QRA at MPA!
Combustion chamber size is critical and is usually larger than initially anticipated. For small calibre weapons, it needs to be at least half the length of the weapon. 'Baffling' with half opened tins is usually successful with the baffles at 90 degrees to each other seeming to work best. The fuel needs to evaporate to burn, so increasing the evaporation surface is a good idea. Something metallic such as a ball of scrunched up locking wire is good, but don't use steel wool as it can ingnite with disasterous consequences!
As previously metioned, the fuel air mix is critical. To weak and you get a 'phut', too strong and it won't ignite. As the evaporation rate of the fuel is critical. combustion chamber warms up, the fuel evaporates faster and the mixture required changes. We found the best solution for a liquid propellant is to use a syringe to carefully measure the required charge. Trial and error testing outside the dining in night environment will determine the correct mixture for the temperature. Also, the syringe encourages vaporization if the mixture is applied under pressure.
Ignition is easy if the mix is correct. But the standard lighter over the hole technique has it's drawbacks. The size of the hole isn't that critical, but too small a hole won't allow enough fuel outside the chamber for instant ignition and too big a hole will reduce the chamber pressure and give quite a lot of 'blowback' - which can be painful! You can 'prime' the orifice with a bit of fuel, but this is an advanced technique. An electric ignition system using a big capacitor and a spark plug works well. But if the person who your cannon is being aimed at decides he doesn't want a 300mph tennis ball hitting his solar plexus and throws his beer at the firer, the system can short giving a near lethal shock to the neck. This will result in the end of the evening for the firer as he's taken to the medical centre! And if using the spark plug method, get a correct size bolt brazed onto the chamber by station workshops in which to screw the plug. If the plug comes out during firing, the collateral damage is spectacular and expensive!
The barrel. Tins of various calibres are readily available. But choice depends on the ammunition availability. Very large calibres (catering vegtable tins/OM15 cans) look impressive and strike fear into the enemy, but ammo choice is a bit limited and muzzle velocity on large weapons is low, so range is limited. Don't use a calibre which relies on a specially made projectile as you probably won't get it back! Join the tins with speed tape, gaffer tape will melt when hot. The end tin of the combustion chamber needs extra securing. Also, for big calibres or advanced fuels consider re-inforcing the combustion chamber with a winding of locking wire and a layer of speed tape.
Ammo: For standard bean tins, tennis balls work well. Fruit and vegtables are good on impact, but the calibres are varible leading to the risk of getting them stuck. having said that, the most effective ammo I have used is partially mouldy oranges! Small plastic footballs are good in large calibre weapons, but can melt if the barrel is hot from repeated use.
Fuel: Lighter petrol is the easiest in my experience. Pressurized butane works well too, but it is a bit tricky to apply. Pyrotechnics are good for closed weapons, but the temptation to set them off outside the weapon is too great. The best fuel I have ever tried is a 75/25 mix of Methyl Ethyl Ketone and Trichlorofluroethane. But be warned - this fuel is only suitable for the very advanced cannoneer!
Aiming system: Laser pointer! it's quick and accurate and also very intimidating. You should see how quick a Blunty can move when he realsies he has a little red dot on his chest!
PS. I take no responsibility for anything tried that I've metioned here. Anything attempted is at your own risk!!!!!!!!
