If they're not paying owners of such stocks fair market value for any such stocks turns in, then they are violating the Fifth Amendment prohibitions against taking private property without just compensation and without due process of law.
If they are giving such compensation, then they are fraudulently misusing taxpayer funds to do so.
Either way, even without taking into consideration the Second Amendment, Massachusetts is acting illegally, here.
Oh for Gawd's sake, what hyperbole.
#1 If a person owns a bump stock, which it is now illegal to possess in MA, a person can voluntarily turn it in to the police. No harm no foul as the voluntary surrender demonstrates compliance with the law.
#2 If a person owns a bump stock, which it is now illegal to possess in MA, a person can voluntarily destroy it on their own without turning it into the government. (I would suggests maintaining documentation showing the destruction.) No harm no foul as the voluntary surrender demonstrates compliance with the law as the individual no longer is in possession of a bump stock.
#3 If a person owns a bump stock, which is now illegal to possess in MA and decides to violate the law and retrain the bump stock then they are subject to arrest. Upon arrest the bump stock becomes evidence of a crime and will be confiscated and held for trial. Having a trial is the very embodiment of "due process".
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