They wouldn't be allowed to confiscate your rifle. This is a thing of general myth. There could never LEGALLY be a confiscation of anything, firearm or otherwise w/o a grandfather clause. The legal term is ex post facto or "after the fact" or "after it's done". In short, it means that if you bought something when it was legal, if it suddenly becomes "illegal" to own one, you can still keep yours. Usually, there is a registration period w/ state or local authorities.
This is how there are (reportedly) Class 3 owners in New York State (no, I don't have evidence of this). They owned whatever before NYS banned them, blah blah, ex post facto, they get to keep whatever. Now, I don't think they can actually shoot their weapons anywhere, but they can OWN them.
A similar case would be if you owned say, a bag of vacuum sealed marijuana that you bought before the marijuana stamp/tax act was initiated and you had a receipt to prove it. You would be in legal possession of the marijuana and you could prove it. Now, whether you could use it is a different matter, but you could still brag, "hey, look at my vacuum sealed bag of legal marijuana...ex post facto!"
