From Wikipedia (Zip Guns commonly referred to as Improvised Firearms):
An improvised firearm is a firearm manufactured by someone who is not a regular maker of firearms (such as a firearms manufacturer or a gunsmith), and is typically constructed by adapting existing materials to the purpose. Called by many names, these improvised firearms range from crude weapons that are as much a danger to the user as the target, to high quality arms produced by cottage industries using salvaged and repurposed materials.
Improvised firearms are more commonly found where legal and commercially produced firearms are unaffordable or strictly controlled. If commercial ammunition is obtainable, then improvised arms will generally be built to fit that ammunition. If commercial ammunition is not available, then muzzle loading black powder designs can still be produced.
Most countries have controls in place that regulate production, sales, and possession of firearms and ammunition (see gun politics for regional information). This means improvised firearms are for the most part illegally produced, which makes their possession and use criminal as well. Improvised firearms are commonly used as tools by criminals and insurgents and therefore often associated with such groups.
The FP-45 Liberator was a pistol manufactured by the United States military during World War II for use by resistance forces in occupied territories.
The FP-45 was a crude, single-shot pistol designed to be cheaply and quickly mass produced. The Liberator had just 23 largely stamped and turned steel parts that were cheap and easy to manufacture. It fired a .45 caliber pistol cartridge from an unrifled barrel. Due to the unrifled barrel, it was intended for very close ambush (1-4 yds). Its maximum effective range was only about 25 feet (less than 8 m). At longer range, the bullet would begin to tumble and stray off course. Because of the low quality, it was nicknamed the "Woolworth gun."