Introduction

I had some parts lying around my bench and I had an idea to make mini combustion cannon. I had a long polystyrene plastic tube that accommodated standard 6mm plastic BBs perfectly and a nicely sized empty tablet pot for the combustion chamber that fit in the hand nicely. The tablet pot also incorporated an end cap that snaps on and off really securely creating a good gas tight seal. This mini cannon works really well with standard 6mm plastic BBs, potatoes, wet paper towel balls or even cotton buds… wearing goggles is essential as the projectiles are small and have a tendency to ricochet everywhere!

Materials used

• Approx. 30mm Ø x 130mm long tablet pot/container (e.g. for calcium carbonate or vitamin tablets) or similar plastic tube with secure but removable gas tight cap/lid. Standard 22mm or 28mm dia. plumbing fittings should also work great.
• 6mm int. Ø (~7.5mm ext. Ø) x 220mm long pipe for the barrel (choose based on desired projectile dia.)
• Piezoelectric sparker from a lighter
• Two thin sowing pins/needles
• Two rings of 22mm Ø plastic pipe
• A couple of lengths of insulated wire
• Solvent weld cement or strong multi-purpose adhesive

Construction of the cannon

I first started with wiring up the sparker. Soldering (heating) directly to the metal base of the sparker can damage the piezoelectric crystal (PZT) inside so it is best to attach a wire by making a small metal contact cup to fit onto the metal base of the sparker. I made the contact cup out of a thin sheet of brass and formed it around the metal base as shown in the photo below. A wire was soldered directly onto the contact cup. The wires leading from the sparker should be approximately the same length of the chamber tube (tablet pot) plus about 50mm. Ignore that the colours of the wires are different in the photo below, I had to use a photo of another spaker from a similar project.

The main chamber consisted of an empty tablet pot (calcium carbonate tablets available from any supermarket).

I drilled a tight fitting hole (7.5mm Ø) in the base of the tablet pot to allow the barrel to fit through, the barrel should be inserted into the chamber tube until it is about 30mm from the lid end of the tube. This allows the barrel length to remain as long as possible yet still allowing the cannon to be compact as a whole. To stop BBs from passing right through the barrel and into the chamber the barrel needs to be slightly  obstructed at the end by using either a small dot of glue on the inside of the tube or a pin pierced through the tube end.

I passed the two wires leading to the igniter through the hole as shown in the photo below. The wires need to travel through the full length of the chamber tube so were pulled through with enough wire to position the sparker at the barrel end of the cannon as seen in the photo below. Multi-purpose glue was used to seal the barrel and wires through this hole and to secure the barrel to the inside of the chamber tube. The igniter was tapped into position by tightly wrapping some electrical tape around the sparker and chamber tube. I also tightly tapped a wedge of cardboard behind the sparker to act as a back stop for the sparker when being depressed. A final layer of duct tape was wrapped around the sparker to tidy it up.

To make the ignition mounting I used a couple of spilt rings of piping cut from a length of 22mm Ø pipe, and fitted one around the other until it fitted snugly in the chamber tube ID. I pierced two sowing pins through the side of this mounting at roughly 100° to each other, checked the spark gap distance worked and then cut and filed the pins flush. I then soldered the two wires from the sparker to each of the pins.

And that’s it – a small mini combustion cannon! Simply spray a very small amount of propellant into the chamber, load a BB into the end of the barrel and you’re good to go.