The Deer gun is an updated version of the Liberator, manufactured by the CIA for use by resistance groups in Vietnam.

According to Wikipedia (which seems accurate in this case):
The Deer gun was made of cast aluminum, with the receiver formed into a cylinder at the top of the weapon. The striker protruded from the rear of the receiver and was cocked in order to fire, and a plastic clip placed there to prevent an accidental discharge, as the Deer gun had no mechanical safety. The grip was hollow and had space for three 9mm rounds, as well as a rod for clearing the barrel of spent cases. The Deer gun lacked any marking identifying manufacturer or user, in order to prevent tracing of the weapons, and all were delivered in an unmarked polystyrene box with a series of pictures depicting the operation of the gun, as well as three 9mm rounds. The Deer gun was loaded by removing the barrel and placing a 9mm cartridge in the chamber. The striker was then cocked, and a small plastic clip placed around the striker to impede the forward motion of the striker to prevent accidental discharge. The barrel was then screwed back onto the receiver. The gun was fired by removing the plastic clip, placing it on the barrel where it would become the sight, and pulling the trigger. At this point the user would take the victim's equipment if opportunity presented itself, and then flee. Later, the user would reload the gun by unscrewing the barrel and ejecting the spent case with the provided barrel rod, and following the outlined procedure.
Using a detachable barrel for loading simplifies the action of the gun, the part most difficult to design and manufacture in an ultra-cheap gun. It would be interesting to see if a version of this gun could be made out of cheap modern plastics, such as the PET plastic used to make soda bottles. A plastic version would have to withstand considerable recoil and pressure, but special cold loaded rounds could potentially compensate (and would be an excellent idea, considering the certain lack of range of this gun). Additionally, a plastic gun could be easily recycled, making it environmentally friendly. If prices were low enough, "arm the homeless" programs might become viable.