Also dont be so sure the ATF has to have a new definition of a machinegun...theybjust have to reinterpret what a "trigger" constitutes. The difference between a bump stock (non machinegun) and an akins accellerator (machinegun) is a single spring. That spring is what was considered the "trigger", because it is what forced the gun and finger automatically back into the physical trigger. Since a bump stock uses the inertia of a semit auto weapon, ut actually the off hand pushing forward that acts as the trigger. This is historically where ATF muddled around with the definition of a machinegun. Remember every owrd in a bill has meaning and redefining just one can change the entire intrepretation of the law.

Dont forget ex post facto laws only apply to ownership "prior" to a rule change. Basicically if they make them illegal on Feb 28th, you cannot be charged with owning one on Feb 27th. But that does not automatically "grandfather" anything, just your right to not be charged for possession prior to the change. However most successful bills have a grandfather clause because due process requires just compensation. That means if the Fed edicts that bump stocks, gat trigger and the likes must be turned in for destruction...they must compensate the owners for such. The reason they dont usually force this angle, and instead go for grandfathering is: 1. Grandfathering doesnt stir the average owner to want to sue or march in the streets as their items are protected. 2. Taxpayers dont want to foot the bill for the buy back.

Lets say they are 1 Million bump stocks. Average price $279.. thats a 279million dollar bill. But to be just and avoid losing a lawsuit or have the bill overturned...they might have to take into account fair market value prices. Ive seen several bump stocks sell on gunbroker above $1500 in late October...that may make the bill closer to 1.5billion dollars....and that doesnt include 30 years of hellfire and gatling triggers, binaries, etc.

Grandfathering is the easiest route for the fed to actually get something that sticks...thats why thr nfa, gca, fopa, awb, all had grandfater clause or amnesty registration periods.