As resident WonderHowTo Potato Gun Expert, I am serving up four excellent tutorial videos. You will notice that while all the videos and articles have slightly different design dimensions, they are fundamentally very similar. I urge you to view and read everything I am serving up.
Then, my final words of advice are simply....choose between a barbeque switch and a taser gun. (Hint: there is a right choice, and there is a wrong choice.)
Last, make your own choice about the proper ratio of chamber to barrel volume. (Hint: I will not be a design bigot in this instance: you are all gifted students!) These are the most critical choices you make in this very important endeavor. I also plan within the next month to post the be all, end all, the mother of all potato gun tutorials. Stay tuned. Wonderhowto has indexed several dozen potato gun tutorials. In this particular case, the length of the video does matter. Rome was not built in one day, and the art of the potato gun cannot be fully absorbed and ingested from a two minute video.