isn’t there a phrase somewhere called ex post facto???
Ex post facto means you can’t be charged for a crime that wasn’t illegal when it was committed.
So, you won’t get in trouble for the weapons you purchased years ago, however, you are still in POSSESSION of them, which is the new crime you are committing after the law has been passed. Or failure to register them, etc. You just won’t be charged for illegally purchasing them. There are plenty of ways for them to get around ex post facto.