“Banning possession of Slide Fires legally purchased would be an unconstitutional ex post facto law.”
In old America that was under that old fashioned Constitution document, that would have been true. New America plays a game of sophistry where the game is that the government can do anything, but you have to make a silly argument that “explains” why its constitutional. They hope to soon abandon that part of the game.
Anyone buying one of those things, at least a Slide Fire, received an opinion from the BATF that the device was perfectly legal.
You’re right though. They are experts in squaring circles. As usual our distinguished senators require schooling in Con Law I if they seek to make something legal, illegal.