“What is an ex post facto law and why is it forbidden in the Constitution?
Laws that reach back in time and make conduct punishable in a way it was not punishable for when it was done are ex post facto laws. Such a law (1) applies to events occurring before its enactment and (2) disadvantages the person affected by it.”
An ex post facto law (rule) in this case would be prosecuting people for owning these devices yesterday. BATFE is not threatening to do that.
This isn’t an ex post facto violation, it’s a Second Amendment violation and an enumerated powers violation.