In the United States, fully automatic firearms, often referred to as machine guns, are heavily regulated. The National Firearms Act (NFA) of 1934 and the Gun Control Act of 1968 impose strict controls on the manufacture, transfer, and possession of machine guns.
Under current law, machine guns that were manufactured and registered before May 19, 1986, are legal to own in most states, provided that the owner has gone through the necessary licensing process and paid a tax. However, the manufacture of new machine guns for civilian use has been prohibited since that date.
In summary, while it is legal to own certain fully automatic guns in the US, they are subject to strict regulations, and new fully automatic firearms cannot be legally manufactured for civilian ownership. Always check your local laws, as regulations can vary by state and locality.