Acytiline is a wonderful propellant, that is, if you trust it. As long as all your connections on the cannon are glued properly, you should not have a problem. It can be picked up at any hardware store in the Bernz O Matic canisters. MethylAcytiline is what is printed on the bottle, which is yellow....

To be serious and help the kid...

Often an easy, effective, but many times overlooked method would be as follows: Simply driving 2 screws into the chamber, keeping the ends close enough to maintain a good spark, and attaching your grill sparker accordingly to the screws.