Romney won't sign a national one, because congress will never put one on his desk. Even with a democrat controlled congress, senate, and white house, they wouldn't do it, it's political suicide and they know it, even more so now.
Also, in DC v Heller, the supreme court said any weapon "in common use" is constitutionally protected, the AR15 is the most popular centerfire rifle in the country, kalashnikovs are very popular as well, so yeah, no way it will stand up to any challenge.