My own personal opinion is that the problem lies in the legal definition of the term "machine gun" which is currently tied to the functioning of the trigger: does it fire more than one bullet with a single pull of the trigger. Attaching a bump stock to a rifle does not make it do this.
What we need instead is a definition of a machine gun based on the rate of fire acheivable by an average person. Over X shots per second and it is a machine gun.