Some examples of the exceptions which allow a U.S. citizen to own a fully automatic firearm include, the possessor isn't a "prohibited person," the fully automatic firearm was made before 1986, and the relevant state laws allow for it, which varies from state to state. The category of "prohibited person" includes anyone who is a felon, has been convicted of any crime punishable by more than a year in prison, is under indictment for any crime punishable by more than a year in prison, is a fugitive, is an illegal user of a controlled substance, has been adjudicated as a mental defective, has been committed to a mental institution, is an illegal alien, has a dishonorable discharge from the military, has renounced their U.S. citizenship, is the subject of a restraining order, or convicted of a misdemeanor crime of domestic violence. This obviously limits the pool of potential legal fully automatic firearm owners significantly, then you add any relevant state laws on top of that and it becomes very difficult to legally possess one. 
 
Let's say you are not a "prohibited person" and live in a state where fully automatic firearms are legal. What does it take to become the owner of a fully automatic weapon? A prospective owner must go through an application process administrated by the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF). This application process requires a federal tax payment of $200 and a thorough criminal background check. This $200 tax payment buys a revenue stamp which is the legal document that allows for the possession of an automatic firearm. Needless to say, owning a fully automatic weapon is not easy or cheap and the pool of eligible applicants is very small. Hence the idea for an accessory to increase the firing speed of a semi-automatic firearm to more closely resemble that of a fully automatic firearm. 
 
The idea for the bump stock was developed by a 63-year-old Marine veteran and Elvis impersonator named Bill Akins. Akins first came up with the idea for the accessory while watching a World War Two documentary in 1996. The documentary showed twin anti-aircraft guns firing at Japanese warplanes from a U.S. Navy ship and he saw the barrels sliding back and forth from the concussive recoil each time they fired. The scene sparked Akins imagination and he began wondering if he could design a device that would mimic that action and turn a semi-automatic rifle into a weapon that mimics the fire rate of a fully automatic weapon. 
 
 
The first iteration of Akins new invention was dubbed the "Akins Accelerator" which he teamed up with a firearms industry businessman to produce. Akins secured two letters from the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF) confirming that the mechanism Akins proposed was legal. He then got to work and poured his life savings into making his idea into a reality. After several years of testing and several prototypes, Akins created a device that successfully turned a semi-automatic firearm into one that could mimic a fully automatic firearm. He did this by creating a rifle stock that contained a spring which utilized the recoil of a semi-automatic rifle to bounce the rifle forward and back into the bearer's trigger finger. 
