According to the Websters Dictionary, a zip gun is a homemade pistol made from a metal tube and a wooden stock, attatched with tape, usually firing .22 caliber rounds. I am making the assumption (although I do not know for sure) that the .22 rounds are chosen because they produce little recoil, not much noise, and they are the easy to set off with a makeshift firing pin, being rim fire cartridges rather than the center fire cartridges usually used for larger ammunition.
It's a kludge used by hooligans because it's easy to build, and hard to trace, and basically disposable. The downside is that they only hold one bullet, and they are notably inaccurate.