However this supreme court said that the magic words 'bump stock' wasn't in the legalisation. Words that didn't even exist until 2003, or thereabouts. The court ignored the legislative text completely.
This is the text of the NFA that has defined what is a machine gun since 1934:
The term “machine gun” means any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot, without manual reloading, by a single function of the trigger. The term shall also include the frame or receiver of any such weapon, any part designed and intended solely and exclusively, or combination of parts designed and intended, for use in converting a weapon into a machinegun, and any combination of parts from which a machinegun can be assembled if such parts are in the possession or under the control of a person.
I'm not a fan of this SCOTUS, but the bump stock ruling was inline with decades of jurisprudence on the topic and the final opinion was fairly unsurprising as a result. It was honestly less of a gun law ruling and more of an executive regulatory procedure one.
A bump stock does not function by a single action of the trigger and does not meet the statutory definition as a result. The ATF rule banning them got struck down because Congress hadn't authorized the ATF to regulate machine guns beyond that specific statutory definition.
Bump stocks are no more a machine gun than a Gatling gun is under the definition that has existed for nearly a century, and the legal status of the latter has been extremely clear for a very, very long time.
If the goal is to treat them as a regulated item, then Congress needs to pass legislation with language that covers them because saying it was already there is simply incorrect. There is a specificity to the language of the NFA that doesn't cover any number of mechanisms. It's been a deficiency of the law since 1934.
If you want to fix that, that first requires understanding exactly what needs fixing.