So, normally I like to write about people using electronics and circuitry in cool ways, but sometimes it’s a lot more satisfying to just build something simple, cool, and big! And evidently the guys at SmarterEveryDay agree. That’s why Destin from SmarterEveryDay created a video capturing the science behind a giant potato gun at 20,000 fps and looks at how to get the best performance out of it based on thermal characteristics and gaseous expansion. You can see about halfway through the video the methods of ignition that he uses, primarily using a BBQ igniter which creates a HV (but extremely low current) spark gap to ignite the gases. Perhaps this goes without saying but this is one of those 100% do not try this at home kind of projects.
The particularly interesting part is the difference in the way the gas expands between being triggered from the rear vs the middle of the combustion chamber. You can see the air being sucked back into the chamber after ignition which is required to maintain and equal pressure. The next step would be triggering the cannon wirelessly using a microcontroller such as the Photon or Electron from Particle, imagine the IoT possibilities!