This will potentially be decided by the Supreme Court of the United States in Garland v. Cargill.
The issue in Cargill is whether a bump stock device constitues a device "designed and intended, for use in converting a weapon into a machinegun" — a machinegun being a weapon which shoots "automatically more than one shot... by a single function of the trigger."
If so, bump stock devices would be regulated as a machinegun under 26 U.S.C. § 5845(b).
As is apparent from the written and oral argument, the terms especially in dispute are:
- "single function,"
- "trigger," and
- "automatically."
When the Supreme Court gives meaning to the various terms in 5845(b), we will then be in a position to answer your question.