SpudFiles

Allright, so I just built a coaxial piston cannon. It has a 4" x 4' chamber and a 1.5" x 5' barrel. The piston is 1/8 inch neoprene with the cloth insert and I fill it with a bycicle pump.
The problem is that the piston won't seal against the barrel. Do I need a spring?
...the instructions from X inventions aren't very helpful.

Point your cannon down so the piston hits the barrel,that seems to work with mine but I am having problems with it.

Your piston is supposed to seal good with the side walls of the chamber and once the piston hits the barrel it's supposed to start filling the chamber up...

Your equalization holes are just letting all the air through... A spring would fix it.

I kind of knew that. Im going to try wrapping it in electrical tape until it is somewhat snug...