What's the Difference Between a Bump Stock and a Semi Automatic Weapon?

The gun control of act of 1968 says that she'll be unlawful for any person to transfer or possess a machine gun. Now here's what's interesting about the bump stock. From the shooters, the trigger pullers perspective, it makes it quite similar to an automatic weapon. The question really boils down to does this mean this statute as a matter of statutory interpretation?