I think that if you don't buy one and keep it ready to use you will be living in fear for your freedom.
I think they can try to make it retroactive but do not know if they can do that legally. They may want to you register it but I don't think they can come take what you already have, unless you can't prove you bought it when it was legal and then they may try and take it anyhow just to make certain it was a legal purchase. Then, of course, you have to try and get it back but they could say that if it was ever confiscated, legal or not, you lose it..............
Here, in nys, I believe they will try and reinstate a proposition that was approved in the house but never made it to the senate, in which any firearm discovered by police, for almost any reason, can be conficated, marked, ballistically tested and registered in the (failed) ballistic data base. You then have to try and recover it.............
This means simply that if you happen to be stopped for a random traffic check and are asked if you have any firearms, the cop can confiscate the firearm if he or she so chooses, and turn it over to the state police for a ballistics check - then you gotta try and get it back. This will continue until or unless a court deems it inappropriate and the practice is either halted or modified.
With a federal gun ban I deriously doubt you will ever get your gun back.
My belife is that even with an 'internal security force, or a civilain security force, or whatever that obamanation wants to call it, there are not enough soldiers to disarm 80 million Americans and I think that if you don't buy one and keep it ready to use in your own defense you will be in fear of losing your freedoms. JMTCW. Mikey.