Explanation for the picture with the cans: It went through four cans with no sign of "stopping". I could test it with more cans however I fear that with added cans increase the chance of the bb diverging from its original trajectory.
Since I made this instructable after building it I will do my best to explain how to build one yourself.
Step 1: Materials
• Airsoft barrel or some other metal tube. (Must not have thin walls!)
• Metal piece (The piece I had was cylindrical at the base and a round ball at the top with a hole through the entire piece)
• Small blocks of wood
• Drill and bits
• Band saw
• Plastic staples or some similar bracket
• Epoxy or high-strength glue (JB weld perhaps?)
• Wire coat hanger
• Pipe cleaner (optional, but its a good idea to keep a clean barrel!)
• Firecrackers and ammunition
Step 2: Construction
The first thing you want to do is prepare the barrel to shoot things. This requires finding a metal piece slightly bigger than the inside diameter of the barrel with a hole through the center. After this is found you have to hammer it into one end of the barrel then hammer a small lip over it to keep it from slipping out. Feel free to shine the barrel in any way you choose.
2.) Making the base
The best way to do this is to find a drill bit the same size as the outside diameter of your barrel. Use this to drill a hole through the block lengthwise at an angle you want. After this is complete cut the block down the center of the hole so it comes out as a semi-triangular piece of wood with a uniform groove down the middle.
Next, epoxy the barrel into the groove and place this entire upper assembly onto a larger block of wood (this larger block acts as a base). Now hammer your plastic staples so that it holds the barrel snug in the grove and secures the small block of wood to the larger. You may need to file down your staples so that it can form a snug fit. Be careful! If you hammer the staples down too much or miss and hit the barrel you risk deforming/damaging the barrel!
Now for the finishing touches. Use a bit of epoxy at the very back of the barrel to prevent it from sliding back after each shot (Yes this small cannon has recoil!). Feel free to stain or finish the wood and shine the barrel.
3.) Shooting your cannon
To shoot your cannon place a firecracker into the barrel fuse down and shake until the fuse comes through the hole. Next, make a small ball out of tissue and use your ram-rod to push that into the barrel. Be sure to pack the tissue nice and tight into the barrel for maximum projectile velocities! Finally place an airsoft bb or similar 6mm ammunition into the barrel. You are now ready to shoot! Get a match and light 'er up!
CAUTION: Though this is a small device it shoots projectiles at very high velocities! Be sure to shoot in a safe direction free of people, animals, and anything you don't wish to shoot!
Be safe: wear your eye and ear protection!