Time Magazine reports:

Starting in the Prohibition era, Congress has restricted automatic weapons and gun ownership at key points. The National Firearms Act of 1934, the first federal gun-control law, did not outlaw automatic weapons, but it made them expensive and difficult to obtain. It required recording all sales in a national registry, as well as the tax, background check, and other requirements listed above.

After the assassinations of President John F. Kennedy and the Rev. Martin Luther King Jr., President Lyndon Johnson spearheaded the Gun Control Act of 1968. It implemented a lot of the fun ownership restrictions still in place today – preventing felons and the mentally ill from buying guns and raising the handgun purchase age to 21, among other things — but it had an important effect on machine guns, as well. The Gun Control Act prohibited selling imported guns to civilians “with no sporting purpose,” and automatic weapons were determined to fall in that category. So after this law was passed in 1968, average gun-owners couldn’t legally buy imported automatic weapons.

The Firearm Owners Protection Act of 1986 was passed to prevent the federal government from creating a registry of gun owners, but it had an amendment tacked on that banned civilian ownership of machine guns manufactured after May 19, 1986. There are reportedly fewer than 200,000 machine guns in the U.S. that meet that criteria, according to Slate.