Congress is prohibited from passing ex post facto laws by clause 3 of Article I, Section 9 of the United States Constitution. I would think that this would extend to the executive branch (BATF) where they are making law (with punishments) with arbitrary regulations at their whim. The time may come soon that this type of executive regulatory lawmaking will be struck down by the judiciary.
Absolutely, this is the heart of the argument period. All they can do is restrict any new firearms like this from being manufactured. They cannot go retroactive with a law like this against those that have already been sold legally.
Ca started this ex post facto tactic in violation of the constitution and they were allowed to do it so now everyone is following suit. The agenda should have been nipped in the bud first time they tried it. But instead everyone turned their heads and have now let it set precedent.