There won't be a national ban, at least one that passes the Senate and House. At the state level it is a different story. In New York, Connecticut, etc., you will not be able to buy one and I suspect there will be great deal of pressure not to allow grandfathered rifles. Though legal gun owners are pretty law abiding, my guess is that where the state wants them to get rid of rifles most will just shut up and put their AR or whatever in the back of the closet. In New York, with the magazine ban, I have heard from several people with now illegal magazines, that that is what they intend to do. In Connecticut people have told me that is also what they intend to do. As one person who I know has several ARs said when we talked about it, "What AR, sold them years ago?".
This is what happened in the 1990s when Connecticut instituted its ban. People were supposed to register them and very few people did.