I think 22rimfire has a pretty good grasp on how this could happen. Remember, it is not really guns that these people hate and want to control...it is gun owners. We are a radical, dangerous breed that needs to be controlled, for our own good and for the welfare of the people in general. Regardless of the ultimate outcome in the court--and that would take several years, during which time Obama will have appointed at least two new anti gun justices, so the outcome is virtually certain--the initial impact of the law would be total disruption and destruction of the semi-auto rifle manufacturing and importing industry.
Consider how many small AR-15 manufacturers there are. Here in Houston, there are at least six that I know of. Move up to the next level and you have companies like DMPS, Windham, and others that will likely go out of business. As to imports, if there is anyone whose sole business is AK's, well...we'll see you at the bankruptcy hearing. Of course, the companies like Magpul, whose business is primarily accessories for TFKBLAAW (The Firearms Known By Liberals As Assault Weapons), will fall onto hard times and then go out of business.
Then you have the hi-cap handgun mags--you know, the things that make 9mm a viable defense round, and that's how the military justifies carrying Berettas. Wow. A Glock 17 with a 10-round mag. What's the point?
And don't think the libs would hesitate for force a generous buy back. Many here talk about "pry from my cold dead hand", but the fact is, we are law abiding citizens, and the prospect of arrest and prison is enough for us to obey the law. Besides, you know how the libs like to spend our money, so they'll be fair about it, and we'll get a small premium over the original purchase price. Plus, some enterprising individuals (as happened in Australia), will scour the landscape for every junk AR or AK they can find to make a quick profit.
Confiscation would be just a little too illegal, but mandatory buy back? I can see it. In fact, I'm thinking of how to practically deal with it. Hmmm. I see a new thread.