A crude weapon made usually in someones basement or garage where all the components of the gun are creatively constructed out of what is available to make the gun function. These are usaully made with such crude materials that often duct tape is even used to hold the thing together. It is usually designed to fire a low power cartridge that will not explode the barrel that is usually just pipe. Most zip guns are of .22 caliber or 12 Ga. shotgun as that is usually what is available in pipe sizes. The zip designation means that a rubberband has been used to propel a firing pin to ignite the cartridge. This method is most commonly used due the ease of finding rubberbands. This is not a hobbiest gun but a crude way to get ones hands on a gun usually to commit a murder or to be used in gang shootings or other such applications.
Guido knowing he could not legally obtain a firearm to kill the squealer that turned him in made a zip gun out of what was in his garage to kill the punk.