One problem I have is the assumption that the stock fundamentally changes the way the firearm works.
The firearm, however, remains a semiautomatic, requiring one pull of the trigger per round fired. What changes is the ability of the average shooter to pull that trigger with greater efficiency.
The rule said that the semiautomatic became an automatic, which it does not. The firing mechanism remains unchanged. The ability of the average shooter to time the pulling of the trigger with greater rapidity is what the stock changes.
If it can be decreed that the ability to depress the trigger beyond a certain cyclic rate of fire makes the firearm a machine gun, then anyone who can accomplish rates of fire beyond those unassisted by mechanical devices will be somehow subject to this rule. that extension would only be logical, considering the firearm's firing mechanism is fundamentally unchanged. Remove a bump stock and the rate of fire is again limited solely by the skill of the shooter and the inherent ability of the firearm to cycle.
Trick shooters, for instance, who have learned to 'fan' a revolver, can fire at theoretical cyclic rates that are incredible without benefit of a bump stock by 'fanning' or other trigger pulling techniques. Would that make revolvers subject to this provision in eventuality? Only a minor modification of the language would be needed.
Even worse is the establishment of precedent which would place the availability and legality of any firearm or component at the sole discretion of an unelected bureaucrat, in direct contravention to the Second Amendment. This is the very sort of power grab the Amendment was intended to discourage.
