From Houston Chronicle:

A federal prosecutor withdrew the unique charge before the trial began for a Houston man accused of owning the device. However, the defense was prepared to call an ATF expert to testify that bump stocks, attachments that cause a rifle to fire more rapidly, do not render a semiautomatic gun a machine gun.

Experts had conflicting views on the matter, said defense attorney Tom Berg. But Rick Vasquez, a retired ATF agent and firearms expert, would have told the court the bump stock did not meet the statutory definition of a machine gun. The prosecution dismissed case, he said, because the government couldn’t prove beyond a reasonable doubt the bump stock was a machine gun.