As @skeptic said, a bump stock. It wasn’t considered a machine gun because it still shot only one shot with the mechanical pull of the trigger. It just made it easier to do so rapidly. The ATF officially classified it as “not a machine gun” in 2010. This is unlike autosears which gave the firearm full auto capability with the mechanical pull of the trigger.
Trump signed an executive order having them re-classified as machine guns. Per wikipedia " The ban went into effect on March 26, 2019, by which owners of bump stocks were required to destroy them or surrender them to ATF, punishable by 10 years imprisonment and a $250,000 fine."
As a side note, it is still possible to bump fire semiautomatic firearms without the use of a bump stock. It is a technique, though it usually requires firing from the hip or other method where the firearm isn’t shouldered. Though since not fired from the shoulder, it is even less accurate and safe.
