If a law is passed and you violate it it by possessing a banned firearm you will certainly get your due process in the form of a criminal trial.
What you do today can't be made illegal tommorow but what you do tommorow can be made illegal today.
Here's an example.
Say you possessed a drug on 2 April and there was no law against said possession.
On 3 April a law goes into effect that bans the possession of said drug.
You still possess said drug on 4 April.
The conduct that will cause you to be charged with a crime is the possession of said drug on 4 April. Just because you legally possessed and used said drug only two days before doesn't mean you can continue to do so after the ban is enacted.
If you were no longer in possession on 4 April you would not be charged with a crime.
If it were ex post facto it would be passed on 3 April and make the conduct on 2 April illegal.
As long as a law makes some future conduct illegal it does not violate the ex post facto clause.
And if they sieze said drug pursuant to a criminal proceeding after the law has been enacted then they don't have to reimburse you for it.
Due process requires a hearing, and in most cases a trial and adjudication by one's peers. Further, confiscation serves no "public use". No gun control act has ever gone so far as to try and confiscate firearms. When police seized weapons in New Orleans after Hurricane Katrina, injunctive relief was immediately sought in Federal court. The Federal court responded by forcing the NOPD to return those firearms to their rightful owners. The people who had firearms confiscated, would be able to sue the NOPD in Federal court, and probably win, if they could prove harm was done to them by an unlawful confiscation.
The reason they had to return the weapons is because they were not confiscated pursuant to any law. Had they been confiscated pursuant to a criminal action the NOPD would have been justified in doing so. As in the law regarding weapons possession was vilated so we took the weapon and chrged the offender. The due process would come as a criminal trial. Most states have laws regarding forfeiture of weapons used in crimes or possessed illegally.
These weapons were not confiscated because the possessors violated the law but because the NOPD did so because they thought they should which is not contained in any law.
Just because no gun control act has ever gone that far doesn't mean it wouldn't be legal if done. The reason no gun control law has ever gone that far is because such an action makes it almost completely unpalatable. The best argument against such laws is not some misguided interpretation of the ex post facto clause, very similar to the one the courts have rejected regarding the lautenburg amendment, but an argument that such a law violates the second amendment.