They will require you to provide firearm possession and "turn in" information on your tax returns. (see same deal on obamacare) If they believe you are in possession of "now illegal" firearms, they will be able to get a warrant to search your home/office, etc. and confiscate whatever they want. If they find you in violation (and they will find ways to do that) you will be prosecuted for filing a false tax return. etc., etc., etc.
SameO, sameO for obamacare or any other tyrannical thing they want to impose on us.
People who voted for these assholes are the biggest fools in the world.