Class III weapons are a result of the National Firearms Act of 1934. They are also referred to as NFA weapons. The Act of 1934 classified fully automatic weapons,those that fire more than once for each trigger pull (Machine Guns), and sawed off shotguns as Class III. These weapons were a favorite of Bonnie & Clyde, Dillinger, Al Capone, and Baby Face Nelson thus the federal government wanted to get rid of them along with all concealable firearms. Franklin D Roosevelt trying to get around the Second Amendment to the Constitution came up with a plan to tax weapons out of existence. The NFA of 1934 established a tax of $200 and a registry of all full auto weapons and sawed off shotguns.
In May of 1986 The Gun Control Act was passed that prevented any civilian ownership of any newly manufactured machine guns. Any Machine Gun (Class 3) manufactured after May 1986 cannot be sold to a regular citizen. However, all registered weapons manufactured before May can continue to be legally bought and sold.
There are only approximately a quarter million "pre-ban" machine guns that exist for trading and selling among an ever growing population of citizens who want to own them for fun shooting or investing. Since 1986 popular Class 3 weapons such as the Uzi and M16 have averaged a return on investment of 14% to 16% annually. The price of Class 3 weapons have never dropped and would likely increase even more in times of recession.