In 1986 federal legislation, called the Firearm Owners Protection Act (FOPA), prohibited the possession of new  machine guns by citizens. This meant that only machine guns made prior to this date in 1986 were lawful to be possessed by citizens (this is still true).

It is called the Forward Assist and is used in the instance of a round not feeding 100% of the way to the point where it seats into the chamber. They're uncommon and in all the years and all the various malfunctions I've had with the weapon, I've never run into an instance where it helped.