With that in mind, this is obviously simply round one in limitting guns/magazines; because I do not believe, for one minute, that the "gun grabbers" are going to settle for this as the "last step". What is being sought is much more than that. What is desired is a federal registry of all existing and new guns, under the guise that it will limit the sales/transfers to criminals, however, as you noted, AL got those guns by killing their owner and taking them, so that also will likely have no impact on the nation's two mass shootings per year.
The talk of universal background checks for all gun/ammo sales is also being done, yet in a bass ackwards way; the Obama proposal is to require use FFL dealers as "middlemen" for all future "private" transfers. This is not enforcable UNLESS all guns are registered, since otherwise how can it possibly be enforced? Yep - creating another "need" for federal registration. The real goal.
The answer, of course, is so simple and we do it now for driving cars; make a state issued, photo ID with the privilege indicated - a drivers license. We test each driver only ONCE, if they pass then issue the ID and let them use that to prove they are legal to drive upon our roadways. The same can be done for NICS background checks, do them ONCE for each adult US citizen and indicate "GUN OK" on that same state issued, photo ID (if they pass the BG check) otherwise the ID is issued w/o that designation. Then ANY sale of guns/ammo at a FFL dealer, public gunshow, Walmart, bait/tackle shop or the typical private sale can simply require that the buyer show their "GUN OK" ID to assure that the sale is legal. No more need for point of sale NICS background checks, which save lots of time and money too.