In most developed countries, only law enforcement and the military are allowed to own fully-automatic firearms. As such, their use is subject to more stringent regulations than other firearms. Among these regulations are the National Firearms Act of 1934 and the Firearm Owners Protection Act of 1986. Under the latter, a person may only purchase a fully-automatic weapon that has been previously registered.
Fully-automatic firearms use a combination of features and accessories that make them very similar to semiautomatic guns. The National Firearms Act of 1934 was a response to the increasing number of Thompson submachine guns (TSMs), machine guns with pistol cartridges. While they were originally designed for use by soldiers in World War I, they quickly became popular with bootleggers and gangsters during the Prohibition era.
While fully-automatic firearms can be highly effective, their use is not necessarily encouraged. For example, military firearms containing machine guns can be highly dangerous. They fire continuously until they run out of ammunition, so they are most likely only found on battlefields. But if you are curious, there are a number of shooting ranges that feature machine guns in their vaults.
As with semi-automatic firearms, fully-automatic firearms are not widely available. Semi-automatic weapons are available to civilians, while fully-automatic weapons are only used by military personnel. Fully-automatic weapons can cost upwards of $15,000, making them less affordable for civilians.